EXCHANGE 


L  A  Study  of  the  Hydrogen  Electrode,  of  the 

Calomel  Electrode,  and  of  Contact 

Potential* 

IL  The  Application  of  the  Hydrogen  Electrode 

to  the  Measurement  of  the  Hydrolysis 

of  Aniline  Hydrochloride,  and  the 

lonization  of  Acetic  Acid  in  the 

Presence  of  Neutral  Salts. 


DISSERTATION 

SUBMITTED  TO   THE   BOARD   OF   UNIVERSITY   STUDIES    OF 

THE    JOHNS  HOPKINS  UNIVERSITY  IN  CONFORMITY 

WITH  THE  REQUIREMENTS  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


BY 

NATHANIEL  EDWARD  LOOMIS 

BAI/TIMORE 
1911 


EASTON,  PA.: 
ESCHKNBACH  PRINTING  COMPANY 


I.  A  Study  of  the  Hydrogen  Electrode,  of  the 

Calomel  Electrode,  and  of  Contact 

Potential* 

IL  The  Application  of  the  Hydrogen  Electrode 

to  the  Measurement   of  the  Hydrolysis 

of  Aniline   Hydrochloride,  and  the 

lonization  of  Acetic  Acid  in  the 

Presence  of  Neutral  Salts* 


DISSERTATION 

SUBMITTED  TO   THE   BOARD   OF   UNIVERSITY   STUDIES   OF 

THE    JOHNS  HOPKINS  UNIVERSITY  IN  CONFORMITY 

WITH  THE  REQUIREMENTS  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


BY 

NATHANIEL  EDWARD  LOOMIS 

BALTIMORE 
1911 


EASTON,  PA.: 
ESCHENBACH  PRINTING  COMPANY 


CONTENTS. 

Acknowledgment 4 

PART  I. 

Introduction 5 

Previous  Work 8 

Theoretical  Discussion 1 1 

Experimental 12 

1.  Apparatus • 12 

2.  Calomel  Electrodes 14 

3.  Hydrogen  Electrodes 22 

4.  Comparison  of  the  Hydrogen  Electrode  with  the  Calomel  Elec- 

trode    26 

5.  Experiments  to  Determine  the  Relative  Efficiency  of  Several 

Solutions  for  Eliminating  Contact  Potential 34 

6.  Summary 40 

PART  II. 

Experiments  with  the  Hydrogen  Electrode 41 

1.  Hydrolysis  of  Aniline  Hydrochloride 42 

2.  Effect  of  Neutral  Salts  upon  the  Dissociation  of  Acetic  Acid.  .  51 

3.  Summary 54 

Bibliography 55 

Biography 58 


251813 


ACKNOWLEDGMENT. 

This  investigation  was  carried  out  under  the  direct  super- 
vision of  Associate  Professor  Acree.  The  author  wishes  to 
take  this  opportunity  to  thank  him  for  the  instructions,  sug- 
gestions, and  assistance  which  he  has  so  freely  given. 

The  author  also  desires  to  express  his  gratitude  to  President 
Remsen,  Professor  Morse,  Professor  Jones  and  Doctor  Swartz 
for  instruction  and  to  Doctor  Turner  and  Doctor  Pfund  for 
many  valuable  suggestions. 


A  Study  of  the  Hydrogen  Electrode,  of  the 

Calomel  Electrode  and  of  Contact 

Potential 


I.    INTRODUCTION. 

For  several  years  we  organic  chemists  have  felt  the  need 
of  some  direct,  very  rapid,  and  accurate  method  for  determin- 
ing the  hydrogen  (also  hydroxyl)  ion  concentration  of  dilute 
solutions.  Such  a  method  would  be  of  special  value  in  the 
study  of  many  organic  reactions  involving,  for  example,  the 
hydrolysis  of  salts  or  the  saponification  of  esters,  the  reac- 
tions of  addition  products  in  cases  of  catalysis  by  hydrogen 
ions,  and  many  others  in  which  the  system  is  gradually  chang- 
ing. 

The  methods  commonly  in  use  heretofore  have  presented 
serious  difficulties  in  their  general  application.  The  conduc- 
tivity method,  for  example,  which  has  had  the  widest  range 


of  application,  rapidly  diminishes  in  accuracy  with  increase 
in  dilution  of  the  solution,  and  furthermore,  the  measurement 
of  small  concentrations  of  acids  in  the  presence  of  other  elec- 
trolytes, especially  the  salts  of  these  acids  with  weak  bases,  is 
almost  impossible.  Special  methods,  such  as  the  use  of 
diazoacetic  ester,  as  suggested  by  Bredig  and  Fraenkel,1  have 
been  used  brilliantly  in  some  cases,  but  are  too  limited  in  their 
range  of  application.  Methods  involving  titration  are  of 
course  useless  in  systems  in  which  a  state  of  equilibrium  is 
established  comparatively  quickly,  for  the  equilibrium  is  dis- 
turbed as  soon  as  any  one  of  the  components  is  removed. 

Although  colorimetric  methods  in  the  hands  of  Veley,  Salm, 
Tizard,  Szyszkowski  and  others  yield  beautiful  results  in 
many  cases,  it  has  been  found  that  neutral  salts  affect  the  colors 
so  greatly  in  other  cases  that  the  method  is  useless  or  at  least 
uncertain. 

The  hydrogen  electrode  has  been  recognized  by  Acree 
as  a  possible  instrument  for  the  solution  of  this  problem.2 
Particularly  suggestive  is  the  work  of  H.  G.  Denham,3  who 
measured  the  degree  of  hydrolysis  of  several  inorganic  salts 
and  of  aniline  hydrochloride.  He  obtained  results  agreeing 
extremely  well  with  those  determined  by  Bredig  by  the  con- 
ductivity method.  Efforts  to  duplicate  his  results  and  other 
work  on  similar  lines  in  this  laboratory  at  first  met  with 
serious  difficulties,  but  these  are  being  gradually  overcome. 

In  view  of  the  extreme  importance  of  any  favorable  re- 
sults in  this  field,  it  seemed  worth  while  to  make  a  careful 
study  of  the  hydrogen  electrode,  with  special  reference  to  its 
constancy,  the  value  of  its  potential  in  different  acids,  the 
ease  of  reproduction,  etc. ;  in  other  words,  to  attempt  to  make 
it  a  standard  electrode  for  use  in  the  same  way  that  calomel 
and  mercurous  sulphate  electrodes  are  used.  To  make  the 
hydrogen  electrode  an  accurate  instrument  for  measuring 

1  Z.  Elektrochem.,  11,  525  (1905);  Z.  physik.  Chem.,  60,  202  (1907). 

2  Desha:  Diss.,  Johns  Hopkins  Univ.,    1909.     This  work  was  begun  in    1907-8, 
and  reported  at  the  Christmas  meeting  of  the  American  Association  for  the  Advance- 
ment of  Science,  in  1908.     See  Science,  30,  624.      Lapworth  has  also  for  some  time 
advocated  an  attempt  in  this  direction. 

3  J.  Chem.  Soc.,  93,  41  (1908). 


hydrogen  ion  concentrations,  the  measurements  must  be  made 
with  a  much  higher  degree  of  accuracy  than  has  ordinarily 
been  done. 

Early  in  the  investigation  it  was  realized  that  much  of  the 
accuracy  of  the  work  would  be  dependent  upon  the  constancy 
and  ease  of  reproduction  of  the  calomel  electrodes  which  were 
used  with  the  hydrogen  electrode.  For  this  reason  the  study 
of  the  calomel  electrode  was  gone  into  very  thoroughly. 

The  method  of  approaching  the  problem  under  considera- 
tion resolved  itself  into  five  lines  of  investigation: 

1.  The  study  of  the  relative  efficiency  of  the  apparatus 
used  by  others  and  of  newer  forms  devised  by  me  to  eliminate 
various  sources  of  error.     The  literature  and  our  own  expe- 
rience in  this  laboratory  have  shown  that  there  is  still  much 
to  be  done  in  this  line.     In  this  connection  I  cannot  refrain 
from  expressing  my  deep  obligation  to  Professor  H.  N.  Morse 
for  his  kindness  in  giving  me  many  valuable  suggestions  when 
I  needed  the  benefit  of  the  rare  knowledge  and  mechanical 
skill  that  have  enabled  him  to  overcome  such  great  difficul- 
ties in  his  own  researches. 

2.  The  study  of   the  calomel   electrodes.     A  large  number 
of  calomel   electrodes   were  prepared  and  measured   against 
each  other  so  that  the  value  adopted  for  the  potential  of  the 
calomel  electrode  was  the  average  of  a  number  and  not  de- 
pendent upon  a  single  electrode. 

3.  The  preparation  of  a  large  number  of  platinum  elec- 
trodes which  were  intercompared  by  the  method  used  for  the 
calomel  electrodes. 

4.  The  direct  comparison  of  the  hydrogen  electrode  with 
the   calomel   electrode,    involving   experiments   to   determine 
the  efficiency  of  various  solutions  in  eliminating  contact  poten- 
tial. 

5.  The  application  of  the  hydrogen  electrode  to  the  deter- 
mination of  the  hydrogen  ion  concentration  of  various  solu- 
tions.    The  hydrolysis  of  aniline  hydrochloride  and  the  effect 
of  neutral  salts  upon  the  dissociation  of  acetic  acid  were  espe- 
cially studied  (see  the  next  part  of  this  dissertation). 


8 

II.    PREVIOUS   WORK 

The  normal  calomel  electrode,  composed  of  mercury,  calo- 
mel and  normal  potassium  chloride  solution,  was  first  used 
and  described  by  Ostwald.1  It  is  described  as  reproducible 
to  within  one  millivolt.  He  gives  the  potential  of  the  electrode 
as  +0.5600  +  o.ooo6(/° — 1 8°).  This  value  was  determined 
by  Rothmund2  by  the  drop -electrode  method. 

A  year  later  Coggeshall3  made  an  extended  study  of  calo- 
mel and  mercurous  sulphate  electrodes  as  to  constancy,  ease 
of  reproduction,  best  form  of  cell,  effect  of  mechanical  disturb- 
ance, etc.  Although  this  work  constituted  the  most  complete 
study  of  standard  electrodes  made  up  to  that  time,  it  left  much 
to  be  desired.  The  measurements  of  potential  were  made 
with  a  Lippman  electrometer,  which  is  a  far  less  accurate 
method  than  that  in  which  a  potentiometer  and  sensitive 
galvanometer  are  used.  He  did  not  use  the  decinormal  calo- 
mel electrodes  at  all.  Although  finding  the  mercurous  sul- 
phate electrodes  to  be  on  the  whole  more  suitable  for  use  than 
the  calomel  electrodes,  he  concluded  that  "bei  Anwendung 
wohl  gereinigter  Chemikalien  und  einer  Vorkehrung  gegen 
Erschiitterungswirkungen,  wie  eine  solche  in  der  partiellen 
Sandfiillung  gegeben  ist,  sind  ohne  Muhe  Normal-Quecksilber- 
Kalomel-Electroden  herstellbar,  deren  electromotorische  Kraft 
von  dem  Normal  Wert  um  nicht  mehr  als  0.0008  Volt  ab- 
weicht,  und  dies  mit  ausserordentlicher  Konstanz." 

Smale4  was  the  first  to  make  an  extended  study  of  the  hy- 
drogen electrode  and  his  work  was  principally  in  connection 
with  the  oxygen-hydrogen  gas  element.  He  concluded  that 
the  material  (platinum,  palladium,  gold  and  carbon)  in  the 
electrode  used  played  no  part  in  the  electromotive  force  of  the 
cell,  provided  that  it  was  not  acte,d  upon  chemically.  The 
surface  and  size  of  the  electrodes,  above  a  certain  limit,  had  no 
effect. 

Wilsmore5  repeated  Smale's  work  and  made  allowance  for 

1  Ostwald-Luther:  "Physiko-Chemische  Messungen,"  3rd  Edition,  p.  441. 

2  Z.  physik.  Chem..  15,  15  (1894). 

3  Ibid.,  17,62  (1895). 
*lbid.,  14,577  (1894). 
*Ibid.,  35,296(1900). 


the  contact  potential  of  the  solutions.  In  regard  to  the  part 
played  by  the  electrode,  he  arrived  at  the  same  conclusions 
as  Smale.  As  his  zero  of  potential  he  adopted  the  potential 
of  the  hydrogen  electrode  toward  a  solution  normal  with 
respect  to  hydrogen  ions.  On  this  basis  he  found  the  value 
of  the  normal  calomel  electrodes  to  be  0.283  volt.  He  calcu- 
lated the  contact  potential  of  a  large  number  of  pairs  of  solu- 
tions, using  data  obtained  by  others,  and  studied  the  electro- 
motive force  of  the  hydrogen-oxygen  gas  battery  and  the 
potential  of  a  large  number  of  metallic  electrodes. 

Richards1  found  that  the  temperature  coefficients  increased 
with  dilution  of  the  solution,  and  that  the  decinormal 
calomel  electrode  is  more  uniform  in  its  behavior  than  the 
electrodes  containing  normal  potassium  chloride  solution.  He 
determined  the  temperature  coefficient  of  the  decinormal  elec- 
trode to  be  0.00079.  In  this  work  he  noticed  certain  gradual 
changes  in  the  potential  of  the  calomel  cells,  especially  of  those 
containing  the  more  concentrated  salt  solutions. 

In  a  later  article2  Richards  and  Archibald  showed  that  this 
gradual  change  is  caused  by  the  formation  of  a  complex  mercuric 
ion  by  the  interaction  of  the  alkali  chloride  and  the  calomel.  This 
decomposition  is  very  slightly  affected  by  light  or  air  but  is 
hastened  by  elevating  the  temperature  or  by  increasing  the 
concentration  of  the  solution  of  the  alkali  chloride.  The  de- 
composition is  almost  negligible  in  o .  i  N  solutions.  This 
work  was  corroborated  by  experiments  carried  on  simul- 
taneously by  Gewecke.3 

Sauer,4  in  an  extended  study  of  various  electrodes,  includ- 
ing the  systems 

Hg— HgCl— N  KC1 

Hg— HgCl^o.i  NKC1 

Hg— HgCl— N  HC1 

Hg— Hgd— o.i  NHC1 

Hg— Hg2S04— N  H2S04 

Hg— Hg2S04-K).  i  N  H2S04, 
concluded  that  the  normal  potassium  chloride-calomel  elec- 

1  Z.  physik.  Chem.,  24,  39  (1897). 

2  Ibid.,  40,  385  (1902). 

3  Ibid.,  45,  685  (1903). 

4  Ibid.,  47,  146  (1904). 


10 

trodes  can  be  made  up  with  a  slightly  greater  degree  of  uni- 
formity than  can  the  decinormal  electrodes,  which  were  found 
to  be  reproducible  to  within  about  0.2  millivolt.  Light  was 
found  to  have  no  effect  upon  the  potential  of  the  calomel 
electrodes.  By  direct  comparison  of  the  normal  and  deci- 
normal electrodes  he  found  that  if  the  former  is  assumed  to 
have  a  value  of  0.560  volt  at  18°,  the  latter  will  have  a  value 
of  0.612. 

Sauer's  observation  as  to  the  greater  uniformity  of  the 
normal  electrodes  has  been  corroborated  by  Lewis  and  Sar- 
gent,1 who  have  also  placed  emphasis  upon  the  purification 
of  materials  and  uniform  methods  of  preparation  of  the  calo- 
mel-mercury paste. 

Palmaer2  determined  by  the  drop-electrode  method  that 
the  absolute  potential  of  the  decinormal  electrode  at  18°  is 
— 0.5732  ±  0.0003.  At  the  same  temperature  the  value  of 
the  normal  electrode  is  — 0.56.  This  gives  a  difference  of 
0.013  between  the  potentials  of  the  normal  and  decinormal 
electrodes.  By  direct  comparison  Sauer  found  the  difference 
at  18°  to  be  0.052.  Since  the  procedure  of  Sauer  is  probably 
a  far  more  accurate  method  for  determining  differences  in  poten- 
tial than  is  the  drop-electrode  method,  it  has  seemed  best  to 
adopt  Wilsmore's  standard  as  the  zero  of  potential,  viz.,  the 
potential  of  the  hydrogen  electrode  toward  a  solution  normal 
with  respect  to  hydrogen  ions.  If  we  use  Sauer's  value  for 
the  difference  between  the  potentials  of  the  normal  and  deci- 
normal electrodeb,  the  value  of  the  decinormal  electrode, 
according  to  Wilsmore's  standard,  becomes  — o .  283  — o  .052  = 
— 0-335  a*  1 8°.  At  25°  the  potential  of  the  decinormal 
electrode  becomes  — 0.335  — 0.0008(25° — 18°)  =  — 0.3406. 

Besides  the  work  briefly  reviewed  above,  there  has  appeared 
an  immense  amount  of  work  involving  the  use  of  the  hydrogen 
electrode,  the  calomel  electrode  or  other  standard  elec- 
trodes. In  this  connection  may  be  mentioned  the  work  of 
Lorenz  and  Mohn3  on  the  neutral  point  of  the  hydrogen  elec- 

1  J.  Am.  Chem.  Soc..  31,  362  (1909). 
3  Z.  physik.  Chem.,  59,  129  (1907). 
3 /&«*.,  60,  422(1907). 


II 

trode;  that  of  Lorenz  and  Bohi1  on  the  electrolytic  dissocia- 
tion of  water;  that  of  Lewis  and  Rupert2  on  the  chlorine 
electrode;  that  of  Naumann3  on  the  electromotive  force  of  the 
hydrogen-cyanogen  gas  element;  that  of  Schoch4  on  the  oxy- 
gen electrode;  and  many  others. 

III.   THEORETICAL   DISCUSSION 

The  theory  of  the  hydrogen  electrode  is  generally  familiar. 
It  will  only  be  recalled  here  that  according  to  Nernst  the  poten- 
tial of  the  electrode  toward  the  solution  in  which  it  is  immersed 
is  dependent  upon  the  pressure  of  the  hydrogen  gas  and  upon 
the  osmotic  pressure  of  the  hydrogen  ions  in  the  solution. 

In  the  comparison  of  a  calomel  electrode  against  a  hydrogen 
electrode  in  a  solution  whose  hydrogen  ion  concentration 
is  H',  we  find  that  if  n  represents  the  observed  electromotive 
force,  TTj  the  potential  of  the  calomel  electrode  against  a  hydro- 
gen electrode  when  immersed  in  a  solution  with  unit  concen- 
tration of  hydrogen  ions,  and  x2  the  contact  potential  between 
the  solutions  of  the  system,  then  the  equation 

R  T 


holds  when  the  hydrogen  gas  is  under  atmospheric  pressure. 
From  this  equation,  when  T  =  (25  +  273°)  we  find  that 


0.0591 

The  value  of  n  is  obtained  by  actual  measurement  with  the 
potentiometer,  nt  is  calculated  from  some  system  in  which 

7T2  and  -7T-  Iog10  H'  are  known  and  it  has  been  previously  meas- 

ured, and  7r2  is  calculated.     The  best  system  for  determining 
the  value  of  itj_  is 

H2—  Pt  |  o.  i  N  HC1  |  o.  i  N  KC1  |  HgCl—  Kg 


in  which  it  can  be  measured,    —  loge  Hr  =  0.0591  X  Iog10 


i  Z.  physik.  Chem.,  66,  733  (1909). 
8  J.  Am.  Chem.  Soc.,  33,  299  (1911). 
3  Z.  Elektrochem.,  16,  191  (1910). 
*  J.  Phys.  Chem.,  14,  665,  719  (1910). 


12 

0.0922,  and  7C2  can  be  calculated  by  some  such  formula  as 
that  of  Planck. 

Because  of  the  difficulty  of  calculating  n2  exactly  in  many 
cases,  attempts  have  been  made  to  eliminate  this  potential 
by  interposing  between  the  two  solutions  in  question  a  satura- 
ted solution  of  some  highly  soluble  salt,  the  two  ions  of  which 
have  nearly  the  same  migration  velocity,  such  as  ammonium 
nitrate,  potassium  chloride,1  and  others.  The  use  of  ammo- 
nium nitrate  for  this  purpose  has  been  advocated  by  Abegg 
and  Gumming,2  who  claim  that  it  practically  eliminates  the 
contact  potential.  On  this  assumption  it  was  used  by  Denham 
in  his  measurements  of  the  hydrolysis  of  aniline  hydrochloride 
and  a  number  of  inorganic  salts.  That  it  does  not  do 
away  entirely  with  the  contact  potential  was  shown  by 
Desha.  Some  measurements  of  my  own  in  this  connection 
will  be  spoken  of  later. 

In  order  to  show  a  change  of  o .  i  per  cent,  in  the  hydrogen 
ion  concentration  of  a  solution,  the  measurements  must  be 
accurate  to  within  0.000025  volt,  and  an  accuracy  of  o.ooooi 
volt  was  striven  for.  The  question  of  the  effect  of  temperature 
also  conies  into  consideration.  Since  i  °  makes  a  difference  of 
0.0008  volt  in  the  potential  of  the  decinormal  calomel  elec- 
trode, the  temperature  had  to  be  kept  constant  to  within 
about  o°.oi. 

iv.  EXPERIMENTAL 

i.  Apparatus 

The  electromotive  force  measurements  were  all  made  with 
a  Leeds  and  Northrup  potentiometer  calibrated  by  the  Bureau 
of  Standards.  All  measurements  were  made  by  the  zero 
method,  that  is,  the  potentiometer  was  adjusted  until  there 
was  no  deflection  of  the  galvanometer.  The  galvanometer 
was  a  Leeds  and  Northrup  special  high-sensibility,  short- 
period  instrument  of  the  Marvin  type.  It  had  a  sensibility 
of  117,  a  period  of  i .  7  seconds,  and  a  resistance  of  215  ohms. 

1  After  this  article  was  in  type  I  learned  of  the  very  important  work  of  Bjerrum 
on  the  use  of  a  saturated  solution  of  potassium  chloride  to  eliminate  contact  potential 
(Z.  Elektrochem.,  17,  389;  Z.  physik.  Chem.,  53,  428).     His  results  agree  very  closely 
with  mine. 

2  Z.  Elektrochem.,  13,  17  (1907). 


13 

As  primary  standards  of  potential  I  used  two  Weston 
standard  cells  kindly  loaned  to  me  by  the  Bureau  of  Standards 
and  calibrated  by  them  from  time  to  time. 

The  apparatus  was  tested  for  leakage  currents  and  ther- 
mal effects  by  making  various  commutations  and  found  to  be 
free  from  them  within  the  limit  of  accuracy  of  the  work.  With 
this  apparatus  measurements  could  be  made  to  within  o.ooooi 
volt  with  a  high  degree  of  accuracy.  The  apparatus  was  tested 
by  measuring  the  electromotive  force  of  one  standard  cell 
against  the  other. 

Value  obtained  by  Bureau  of  Standards  =  1.01892. 

Value  measured  on  the  potentiometer  =  1.01892. 

Value  after  correcting  in  accordance  with  calibration  of 
potentiometer  =  1.01892. 

The  experiments  were  all  carried  out  at  25°  C.  The  ther- 
mometer used  in  the  bath  was  compared  about  once  a  week 
with  a  Beckmann  thermometer,  which  was  in  turn  compared 
to  within  o°.oo2  with  two  mercury  thermometers  calibrated 
by  the  Bureau  of  Standards  to  about  o°.ooi. 

For  a  constant- temperature  bath  there  was  used  a  glass  aqua- 
rium 36  X  1 6  X  15  inches,  partially  filled  with  oil,  as  illustrated 
in  Figs,  i,  7,  ja.  At  first  an  attempt  was  made  to  use 
an  ordinary  water  bath,  then  an  oil  bath  immersed  in  a  water 
bath,  but  electric  leakage  currents  in  both  cases  made  it 
necessary  to  adopt  the  oil  bath.  The  oil  used  was  a  light 
lubricating  oil,  very  transparent,  nearly  colorless  and  odor- 
less, and  free  from  sulphur,  as  was  shown  by  the  mercury 
test.  The  heating  was  accomplished  by  an  electric  light, 
(L)  in  Fig.  7,  which  was  regulated  by  a  relay  and  thermo- 
regulator.  A  fan  stirrer  situated  at  one  end  of  the  aquarium 
drove  the  oil  down  and  under  a  glass  plate  placed  four  inches 
above  the  bottom  to  the  further  end  of  the  bath,  where  oil 
rose  and  returned  through  the  thermoregulator  and  above 
the  plate  to  the  stirrer.  The  glass  plate  also  served  as  a  sup- 
port for  the  apparatus  used.  Under  the  glass  plate  is  a  cool- 
ing coil  not  shown  in  the  figures.  The  thermoregulator  was 
a  toluene  grid  of  the  type  in  use  in  this  laboratory.  The  tem- 
perature regulation  was  constant  to  within  o°.oi. 


The  hydrogen  used  for  the  hydrogen  electrode  was  generated 
electrolytically  from  ten  per  cent,  sodium  hydroxide  solu- 
tion, as  shown  in  (7),  Fig.  7,  nickel  electrodes  being  used. 
A  current  of  about  one  ampere  was  generally  employed.  To 
remove  the  last  traces  of  oxygen  from  the  hydrogen,  it  was 
passed  through  an  electrically  heated  tube  containing  palladium 
asbestos.  The  tube  (M  in  Fig.  7  and  A  in  Fig.  ja)  was 
made  of  Jena  combustion  tubing,  6  mm.  in  diameter,  and  was 
fitted  with  a  mercury  trap  at  one  end.  It  was  covered  with 
a  layer  of  asbestos  and  inserted  in  a  close  fitting  brass  tube 
(N  in  Fig.  7)  having  an  inner  diameter  of  9  mm.  Around 
the  brass  tube  were  wrapped  60  ohms  of  "No.  38  Nichrome" 
ribbon,  the  layers  being  insulated  from  each  other  by  asbes- 
tos paper.  When  this  coil,  in  series  with  73  ohms  (a  32  and 
a  1 6  c.  p.  lamp  in  parallel),  was  connected  to  the  no- volt  city 
circuit  a  temperature  of  170°  C.  was  obtained.  A  slow  flow  of 
gas  through  the  tube  had  no  appreciable  effect  upon  the  tem- 
perature. 

From  the  palladium  asbestos  tube  the  hydrogen  passed 
through  a  washing  apparatus  (B,  in  Figs.  7  and  70), 
which  contained  the  same  solution  as  that  used  around  the 
hydrogen  electrode. 

The  general  arrangement  of  the  bath,  motor,  hydrogen 
generator  and  potentiometer  is  shown  in  the  accompanying 
photograph,  Fig.  i.  Since  the  metallic  tank  in  which  the 
aquarium  is  placed  hides  the  lower  part  of  the  apparatus, 
a  more  detailed  diagram  of  it  is  given  later  in  Fig.  7. 

2.  Calomel  Electrodes 

(a)  The  Preparation  of  the  Materials  used  in  the  Calomel 
Electrodes.  Purification  of  Mercury. — About  thirty  pounds 
of  mercury  were  purified  by  washing  it  with  3  per  cent,  nitric 
acid  for  24  hours  in  a  modified  form  of  Desha's  mercury  ap- 
paratus, illustrated  in  Fig.  2.  In  this  apparatus  three  im- 
portant changes  have  been  made  in  the  form  described  by 
Desha.  In  his  apparatus  the  mercury,  descending  from  the 
reservoir  (A),  entered  a  trap  and  overflowed  into  the  tube 

i  Am.  Chem.  J.,  41,  152  (1909). 


K) 

tvo 


15 

leading  to  the  nitric  acid.  This  caused  a  "dead5'  space  which 
perhaps  slightly  decreased  the  efficiency  of  the  washing.  In 
the  apparatus  shown  hi  the  figure  this  dead  space  has  been 
eliminated  by  the  substitution  of  the  bulb  (#),  which  keeps 
all  the  mercury  in  circulation.  A  more  important  change  has 
been  made,  however,  in  the  method  of  spraying  the  mercury 
into  the  acid  solution.  In  Desha's  apparatus  the  mercury 
streamed  through  holes  in  a  glass  bulb.  Hildebrand1  sug- 
gested the  use  of  muslin  for  spraying  mercury  into  acid.  Pro- 
fessor Morse  and  Dr.  W.  W.  Holland  in  this  laboratory  have 
improved  upon  muslin  by  using  No.  21  bolting  silk,  which  has 
an  extremely  fine  mesh.  This  practice  has  been  incorporated 
in  this  apparatus,  the  silk  being  tied  with  silk  thread  to  the 
glass  tube  at  (C). 

Another  change  consists  in  the  substitution  of  a  tube  (E) 
of  3  mm.  internal  diameter  for  the  i  mm.  tube  which  Desha 
used  to  draw  the  mercury  from  the  bottom  to  the  top  of  the 
apparatus.  This  change  doubled  the  rapidity  of  washing. 
With  the  i  mm.  tube  three  minutes  were  required  for  100  cc. 
of  mercury  to  circulate  through  the  apparatus;  only  one  and 
a  half  minutes  are  required  with  a  larger  tube.  This  form  of 
apparatus  makes  possible  the  electrolysis  of  the  mercury  simul- 
taneously with  the  washing,  the  column  of  mercury  above 
the  silk  being  made  the  anode  and  a  piece  of  platinum  foil 
introduced  at  (D)  the  cathode.  The  platinum  cathode  was 
enclosed  in  a  silk  bag  to  prevent  the  deposited  metal  from 
dropping  back  into  the  solution.  After  being  washed  about 
500  times  through  nitric  acid  the  mercury  was  rinsed  with 
water  and  allowed  to  stand  under  concentrated  sulphuric 
acid  until  used. 

The  mercury  thus  purified  was  distilled  four  times  in  a  cur- 
rent of  air  in  an  electrolytically  heated  Hulett  vacuum  still. 
An  attempt  was  made  to  determine  the  relative  purity  of  the 
different  samples  of  mercury  by  Hulett's2  electromotive- 
force  method  by  using  the  sample  distilled  four  times  as  a 
standard.  Although  by  this  method  one  part  of  zinc  in  io10 

1  J.  Am.  Chem.  Soc.,  31,  933  (1909). 

2  Phys.  Rev..  21,  388  (1905), 


i6 

parts  of  mercury  can  be  detected,  no  difference  could  be  ob- 
served between  the  samples  distilled  one,  two,  three  and 
four  times,  respectively.  I  understand  that  no  differences 
can  be  detected  in  the  electromotive  force  of  standard  Weston 
cells  made  in  the  Bureau  of  Standards  from  different  samples 
of  mercury  purified  in  a  manner  similar  to  mine. 

Mercury  distilled  three  times  was  used  for  the  preparation 
of  calomel  and  in  the  calomel  electrodes. 

Preparation  of  Calomel. — Pure  calomel  was  prepared  by 
dissolving  thrice-distilled  mercury  in  redistilled  nitric  acid, 
an  excess  of  mercury  being  present,  then  pouring  this  solution 
into  dilute  nitric  acid  and  precipitating  the  mercurous  chlor- 
ide by  the  addition  of  hydrochloric  acid  with  constant  stirring. 
The  calomel  was  filtered  and  washed  thoroughly  to  remove 
hydrochloric  and  nitric  acids.  It  was  then  shaken  with  suc- 
cessive portions  of  water  for  several  days  in  a  shaking  ma- 
chine, then  with  a  dilute  solution  of  potassium  chloride  and 
finally  with  a  o .  i  N  potassium  chloride  solution  made  by  dis- 
solving 7.456  grams  of  ignited  recrystallized  potassium  chlor- 
ide in  conductivity  water  and  diluting  the  solution  to  one 
liter.  During  the  entire  procedure  free  mercury  was  present 
and  the  calomel  was  prote£ted  from  the  light  by  the  use  of 
bottles  painted  black. 

(b)  Form  of  Cell. — Some  preliminary  experiments  were 
next  carried  out  to  determine  the  relative  value  of  different 
forms  of  cells  for  use  in  comparing  the  hydrogen  electrode 
against  the  calomel  electrode.  Four  different  types  were  used 
and  the  one  finally  decided  upon  as  most  efficient  and  best 
meeting  our  requirements  is  that  shown  in  Fig.  3.  The  cell 
consists  of  a  tube  (A)  about  2  cm.  in  diameter  and  15  cm. 
high,  into  the  bottom  of  which  is  sealed  a  platinum  wire  with 
which  contact  is  made  through  a  side  arm  (B)  containing 
mercury.  Over  the  top  of  the  cell  fits  a  cap  (C)  with  a  ground- 
glass  joint  (D).  The  cap  is  attached  above  to  a  reservoir 
(E),  through  which  liquid  can  be  poured  into  the  cell.  The 
side  tube  (F)  is  about  one  cm.  in  diameter.  The  stopcock 
(H),  in  which  the  side  tube  of  the  cell  terminates,  serves  to 
prevent  the  diffusion  of  liquids  into  the  cell'  diffusion  is  still 


more  prevented  when  (H)  and  accessible  portions  of  the  wide 
side  tubes  are  packed  with  glass  or  quartz  wool.  It  terminates 
in  the  ground  joint  (7)  (about  22  mm.  long)  by  which  one  cell 
may  be  connected  with  another,  as  shown  in  the  figure,  or 
with  the  hydrogen  electrode  apparatus. 
The  advantages  of  this  form  of  cell  are : 

1.  The  ground-glass  cap  can  readily  be  removed  to  allow 
free  access  to  the  inside  of  the  cell  for  cleaning  and  filling. 

2.  The  mercury  contact  through  the  side  arm  prevents  dis- 
turbances of  the  calomel-mercury  paste  such  as  are  likely  to 
occur  when  contact  is  made  by  a  tube  running  down  inside  of 
the  cell. 

3.  The  stopcock  and  reservoir  above  the  cell  permit  the 
rinsing  out  of  the  cell  with  fresh  solution  when  there  is  any 
suspicion  that  impurities  have  diffused  into  the  side  tube. 

4.  The  stopcock  at  the  end  of  the  side  tube  in  a  great  meas- 
ure prevents  diffusion. 

5.  The  large  diameter  of  the  side  tube  gives  a  low  resistance 
to  the  cell. 

6.  The  cell  can  be  immersed  entirely  in  the  oil  bath,  only 
the  reservoirs  and  stopcocks  being  above  the  oil. 

This  was  the  form  of  cell  adopted  for  comparison  with  the 
hydrogen  electrode.  In  order  that  the  value  used  for  the 
potential  of  the  standard  electrode  might  not  be  dependent 
upon  one  electrode  only,  a  battery,  Figs.  4  and  4.0-,  of  ten 
cells  sealed  together  was  prepared,  so  arranged  that  the  com- 
parison cell  could  be  checked  against  this  battery.  These 
ten  cells  were  of  the  same  type  described  above  except  that  the 
side  tube  was  left  off  and  instead  five  electrodes  were  sealed 
to  each  side  of  a  central  tube  (A).  This  central  tube  was 
turned  up  at  each  end  and  ground  to  fit  the  ground  joint 
(B)  of  the  electrode  (C),  or  (/)  of  the  comparison  electrode, 
shown  in  Fig.  3;  the  comparison  electrode  could  therefore 
be  directly  checked  against  any  of  the  ten  electrodes.  Any 
defective  electrode  can  be  emptied,  cleaned  and  refilled  at  any 
time  without  opening  the  other  nine. 

(c)  Filling  the  Cells. — Before  the  cells  were  filled  they  were 
first  cleaned  with  chromic  acid  and  then  washed  thoroughly 


i8 

with  water.  The  platinum  wires  in  the  bottom  of  the  cells 
were  coated  with  mercury  by  the  electrolysis  of  mercurous 
nitrate  solution.  The  cells  were  then  filled  with  a  strong  solu- 
tion of  potassium  hydroxide,  allowed  to  stand  24  hours,  washed 
with  water  and  treated  successively  with  chromic  acid,  water 
for  2  days,  a  solution  of  potassium  hydroxide  for  12  hours, 
dilute  nitric  acid  for  2  hours,  water  and  finally  alcohol. 

In  making  up  the  cells  the  side  arms  (E)  were  first  filled  with 
mercury,  that  washed  in  nitric  acid  being  used  for  this  pur- 
pose. About  2  cc.  of  the  mercury  distilled  3  times  was  then 
placed  in  the  bottom  of  each  cell  and  on  top  of  this  about  4 
cc.  of  the  calomel-mercury  paste.  The  apparatus  was  then 
filled  with  a  decinormal  potassium  chloride  solution  previously 
saturated  with  calomel.  Recrystallized  and  ignited  potas- 
sium chloride  and  conductivity  water  were  used. 

In  the  earlier  experiments  thick  stopcock  grease  was  used 
for  the  ground-glass  joint  (D)  between  the  cap  and  the  cell. 
The  cells  were  then  painted  over  entirely  with  a  black  varnish, 
especial  care  being  taken  to  get  a  good  coating  of  paint  over  the 
exposed  edge  of  the  ground-glass  joint.  The  paint  was  intended 
for  the  double  purpose  of  protecting  the  calomel  from  light 
and  of  preventing  the  oil  from  dissolving  the  grease  in  the 
ground-glass  joint.  In  spite  of  this  precaution  considerable 
difficulty  was  at  first  experienced  in  the  creeping  of  the  oil 
into  the  cell.  Later  this  difficulty  was  obviated  by  the  use  of 
sealing  wax  in  the  ground-glass  joint.  This  accomplished  the 
purpose  desired  but  was  rather  inconvenient  to  use,  as  the 
joint  had  to  be  heated  upon  making  up  or  taking  down  any 
cell.  There  was,  furthermore,  the  attendant  danger  of  crack- 
ing the  apparatus,  which,  however,  never  occurred.  Another 
form  of  joint  has  been  planned  which  should  obviate  this 
difficulty.  It  is  sketched  in  Fig.  5.  It  differs  from  that 
shown  above  in  having  the  cap  fit  into  the  top  of  the  cell 
and  in  having  a  mercury  trap  around  the  base  of  the  joint 
at  (A).  Sealing  wax  will  be  used  to  close  the  ground  joint 
at  the  exposed  edge  (B) . 

(d)  Measurements  with  the  Calomel  Electrodes. — The  bat- 
tery of  calomel  electrodes  was  made  up  3  times  in  all.  During 


19 

the  first  2  times  difficulty  was  experienced  in  keeping  the 
oil  out  of  the  cell  and  individual  cells  had  to  be  renewed  oc- 
casionally. The  third  time  sealing  wax  was  used  in  the  joints 
and  this  proved  efficient  in  protecting  the  cell  from  oil.  Only 
the  results  of  the  third  series  are  given  in  detail.  The  first 
2  series  of  readings  are  briefly  summarized. 

The  battery  was  first  made  up  on  November  28,  1910.  Four 
days  later  the  maximum  variation  of  the  electrodes  was  0.05 
millivolt.  By  December  16,  2  of  the  10  electrodes  were  0.14 
and  o .  1 6  millivolt,  respectively,  from  the  mean  of  the  other 
eight,  which  differed  from  each  other  by  a  maximum  varia- 
tion of  only  0.06  millivolt.  These  two  electrodes  were  emp- 
tied, cleaned  out  and  made  up  fresh.  The  potential  of  these 
two  on  December  19  agreed  closely  with  that  of  the  others, 
there  being  a  maximum  variation  of  o.io  millivolt.  By 
January  13  the  maximum  variation  had  increased  to  0.17 
millivolt.  The  battery  was  taken  apart  and  traces  of  oil 
were  found  in  all  the  electrodes. 

The  battery  was  made  up  a  second  time  on  January  24, 
1911.  On  January  26  there  was  a  maximum  variation  of 
0.09  millivolt,  which  increased  to  0.17  by  February  10.  The 
caps  of  the  cells  were  then  sealed  on  with  sealing  wax.  On 
February  13  the  maximum  variation  was  o.io,  which  grad- 
ually increased  to  0.16  by  March  7.  The  battery  was  then 
taken  apart  and  cleaned. 

On  March  9  the  battery  was  made  up  for  the  third  time  and 
the  caps  sealed  on  with  sealing  wax.  In  the  following  tables 
the  calomel  electrodes  are  designated  by  the  numbers  i  to 
12.  In  cleaning  the  battery  Cell  No.  6  was  accidentally  broken 
and  after  making  up  the  battery  it  was  found  that  the  poten- 
tial of  No.  7  could  not  be  read  because  of  oil  that  had  crept 
in  between  the  platinum  wire  and  the  mercury  in  the  side 
tube;  only  the  potentials  of  i,  2,  3,  4,  5,  8,  9  and  10  are  given. 
The  readings  are  expressed  in  hundredths  of  a  millivolt.  Cell 
No.  i  is  taken  as  the  standard  electrode  and  considered  posi- 
tive and  the  potentials  of  the  other  electrodes  are  referred  to 
it.  A  negative  sign  before  the  reading  of  any  electrode  means 
that  that  electrode  is  really  positive  with  respect  to  No.  i. 


20 


If,  for  example,  we  write  No.  i  :  No.  9=4-2,  No.  9  has  a 
potential  of  two-hundredths  of  a  millivolt  less  than  No.  i; 
whereas  if  we  write  No.  i :  No.  9  =  — 2,  No.  9  has  a  potential 
two-hundredths  of  a  millivolt  greater  than  No.  i. 


Date 

2 

3 

4 

5 

8 

9 

10 

Mar. 

10 

II 

I 

—4 

6 

2 

2 

—23 

ii 

—  5 

9 

—  15 

13 

—  4 

6 

5 

7 

I 

2 

2 

H 

—  3 

7 

4 

7 

I 

2 

—  3 

15 

—  3 

7 

5 

7 

2 

3 

2 

16 

—  2 

6 

4 

6 

2 

3 

O 

i? 

2 

5 

4 

5 

0 

0 

I 

18 

2 

5 

2 

4 

O 

0 

0 

20 

5 

3 

0 

i 

o 

o 

I 

21 

5 

2 

I 

0 

0 

—  I 

0 

22 

—  5 

5 

0 

3 

I 

I 

5 

23 

—  5 

5 

0 

2 

o 

o 

4 

24 

—  5 

3 

0 

0 

—4 

—  I 

3 

25 

—  5 

3 

3 

0 

—4 

0 

3 

27 

—  i 

4 

2 

0 

2 

0 

5 

29 

—  i 

0 

—4 

0 

—5 

0 

5 

30 

—  i 

0 

—6 

O 

—5 

0 

4 

31 

—  i 

i 

—3 

4 

__  ^ 

6 

7 

Apr. 

I 

3 

3 

2 

4 

—3 

5 

5 

3 

2 

—  i 

—7 

o 

—7 

3 

4 

4 

0 

0 

—4 

3 

—4 

4 

6 

5 

0 

0 

5 

3 

—5 

5 

5 

6 

-  4 

o 

—5 

2 

—6 

4 

5 

7 

—  5 

—3 

—7 

I 

—8 

3 

5 

8 

—  4 

0 

—6 

I 

—8 

2 

5 

10 

o 

2 

—5 

I 

—5 

5 

8 

ii 

0 

—4 

—5 

O 

—7 

5 

4 

12 

0 

—6 

—6 

I 

—6 

4 

3 

13 

0 

—4 

—5 

3 

—6 

6 

6 

15 

0 

2 

4 

5 

—5 

7 

8 

26 

0 

0 

—5 

3 

—5 

9 

10 

27 

—  I 

—4 

—6 

2 

—7 

6 

8 

21 


11 

(made  up) 

Apr. 

28 

—  i 

—3 

,   ,  >-, 

i 

—  8 

5 

5 

—13 

May 

i 

2 

—  i 

—  4 

2 

o 

5 

5 

—  6 

2 

2 

—3 

—  6 

I 

—  9 

5 

5 

—  8 

3 

2 

—3 

—  6 

I 

—  8 

5 

5 

_     *+ 

4 

2 

—3 

10 

0 

—  10 

2 

5 

—  6 

5 

^ 

—4 

12 

I 

—  ii 

3 

5 

—  7 

6 

0 

—4 

II 

I 

IO 

3 

5 

~  5 

8 

0 

—  i 

—  9 

3 

—  7 

6 

10 

—  i 

9 

3 

0 

5 

4 

—  4 

7 

ii 

0 

(e)  Conclusions  Regarding  the  Electrodes  o .  i  N  KCl-HgCl— 
Hg. — An  examination  of  this  series  of  readings  shows  that 
decinormal  calomel  electrodes  can  be  prepared  which  after 
the  first  4  or  5  days  will  vary  from  each  other  by  not  more 
than  a  tenth  of  a  millivolt,  the  majority  being  in  even  much 
closer  agreement.  This  agreement  lasts  for  about  3  weeks, 
after  which  there  is  a  gradual  increase  in  the  maximum  varia- 
tion to  0.14  millivolt  after  2  months.  Essentially  the  same 
facts  were  observed  in  the  first  2  series  of  readings  made 
when  the  battery  was  set  up  on  November  28,  1910,  and  again 
on  January  24,  1911. 

The  average  constancy  of  the  mean  of  the  seven  electrodes 
is  very  good.  On  March  13  the  mean  potential  of  the  seven 
was  2.1,  on  May  9  it  was  2.3,  when  referred  to  electrode  No.  i. 

The  constancy  of  the  electrodes  is  shown  by  their  variations 
from  the  mean  at  the  beginning  and  at  the  end  of  the  ex- 
periment : 

Date 

Mar.    13 
May     9 

+  7       —6     -10  +3       —5   +5     +13  =  Total 

change 

The  average  change  in  the  potential  of  the  individual  elec- 
trodes with  respect  to  the  mean  potential  was  0.07  millivolt 
in  2  months.  That  a  gradual  change  in  the  potential  of  all 
the  cells  did  not  occur  is  shown  by  the  fact  that  Cell  No.  ii, 
made  up  on  April  27,  agreed  so  closely  with  the  others. 


2 

3 

4 

5 

8 

9 

10 

—6 

4 

3 

5 

1 

0 

4 

i 

2 

—  7 

2 

—6 

5 

9 

22 

The  average  daily  change  in  the  potential  of  the  electrodes 
was  very  nearly  o.oi  millivolt,  although  larger  variations 
often  occurred. 

Lewis  has  emphasized  the  necessity  of  preparing  the  calo- 
mel-mercury paste  for  the  calomel  electrode  under  uniform 
conditions.  To  test  the  effect  of  a  different  sample  of  calo- 
mel, 2  electrodes  were  made  up  from  calomel  prepared  about 
6  months  before  that  used  in  the  battery.  Before  use  it  was 
shaken  out  with  fresh  o.i  N  potassium  chloride  solution. 
No  difference  in  the  potential  of  the  electrodes  due  to  the 
change  in  material  could  be  detected. 

A  few  experiments  were  made  to  test  the  effect  of  light  on 
the  potential  of  the  electrodes.  As  a  rule  the  cells  were  painted 
black,  but  when  left  unpainted  and  exposed  to  the  electric 
light  of  the  bath  and  the  diffused  light  of  the  room  no  change 
in  potential  could  be  noticed. 

j.  Experiments  upon  the  Constancy  and  Accuracy  of  Reproduc- 
tion of  the  Hydrogen  Electrode 

In  the  course  of  the  experiments  with  the  hydrogen  elec- 
trode there  were  used  19  platinum  electrodes  designated  by 
the  numbers  i  to  19.  Of  these  electrodes  three,  Nos.  5,  6  and 
7,  had  been  prepared  by  Desha.  Nos.  5  and  6  were  sheet  elec- 
trodes of  the  same  style  as  Nos.  1-4  and  8-19,  inclusive.  They 
were  made  up  of  sheet  platinum,  1X2  cm.  in  size,  welded  to 
a  piece  of  platinum  wire  i .  5  cm.  long,  which  was  sealed  into 
the  bottom  of  a  glass  tube.  Contact  was  made  with  the  elec- 
trode by  a  small  quantity  of  mercury  in  the  bottom  of  the 
glass  tube.  No.  7  was  a  Cottrell  gauze  electrode1  made  by 
weaving  together  with  fine  platinum  wire  the  edges  of  two 
platinum  wire  "baskets"  from  a  broken  Linneman  fraction- 
ating column.  This  gauze  sphere  was  sealed  to  the  end  of 
a  piece  of  glass  tubing,  through  which  the  hydrogen  passed 
from  the  washing  apparatus.  Contact  was  made  with  this 
electrode  by  a  platinum  wire  running  through  the  tube  of 
the  mercury  trap  of  the  washing  apparatus.  Desha  had  coated 
the  platinum  gauze  with  gold  and  then  with  indium.2 

1  Robertson:  J.  Phys.  Chem.,  11,  437.     Schmidt  and  Finger:  Ibid.,  12,  406. 
a  Ostwald-Luther,  p.  438.     Cottrell,  Lewis:  private  communications. 


23 

I  first  used  4  sheet  platinum  electrodes,  Nos.  1-4,  inclu- 
sive. After  the  electrodes  had  been  thoroughly  cleaned, 
they  were  platinized  with  a  solution  made  of  very  pure  plat- 
inum chloride  obtained  from  Heraeus  and  quite  free  from 
iridium  and  other  metals,  which  often  occur  in  platinum 
chloride.  Electrodes  5,  6,  and  7  were  also  replatinized  at  this 
time  with  the  same  solution.  In  platinizing  the  electrodes 
no  special  precautions  were  used.  A  potential  of  2.5  volts 
was  generally  employed  and  the  electrodes  adjusted  until 
there  was  a  fairly  rapid  evolution  of  gas.  The  current  was 
commutated  each  5  minutes  until  a  good  coating  of  platinum 
black  had  been  deposited.  The  behavior  of  the  electrodes 
appeared  to  be  independent  of  the  thickness  of  the  platinum 
coating,  provided  it  was  so  thick  that  the  electrodes  did  not 
appear  gray. 

When  platinized,  the  electrodes  were  washed  with  water 
and  then  connected  i .  5  hours  as  cathodes  in  the  electrolysis 
of  dilute  sulphuric  acid.  They  were  finally  boiled  several 
hours  in  water  and  were  then  ready  for  use. 

In  the  earlier  experiments  the  following  arrangement  was 
used  for  comparing  the  electrodes:  Several  electrodes  we^re- 
passed  through  a  rubber  stopper  which  fitted  the  outer  jacket 
of  a  freezing-point  apparatus,  and  this  tube  was  filled  with 
the  acid  solution  until  the  electrodes  were  about  three-fourths 
immersed.  The  hydrogen  was  introduced  into  the  solution 
through  a  central  tube  drawn  out  to  a  capillary.  By  this 
arrangement  the  hydrogen  could  not  be  bubbled  directly 
against  the  electrodes  and  hence  they  were  rather  slow  in 
coming  to  equilibrium.  The  gauze  electrode  No.  7  could  not 
be  compared  with  the  others  as  its  shape  prevented  its  intro- 
duction through  the  rubber  stopper.  It  could  be  compared 
with  any  one  of  the  other  electrodes,  however,  and  numerous 
experiments  proved  it  to  have  approximately  the  same  poten- 
tial. These  measurements  with  the  gauze  electrode  are  dis- 
cussed in  the  next  two  sections  of  the  experimental  work. 

In  this  preliminary  work  two  comparisons  of  Electrodes 
!>  2,  3>  5  and  6  were  made.  In  each  case  o.  i  N  hydrochloric 
acid  was  used  as  the  electrolyte.  In  the  first  experiment 


24 

the  electrodes  reached  a  constant  potential  after  24  hours. 
Four  of  the  five  electrodes  showed  a  maximum  variation  from 
each  other  of  0.08  millivolt.  Electrode  i,  however,  varied 
by  0.19  millivolt  from  the  mean  of  the  others.  It  was  found 
to  be  oily,  so  it  was  washed  with  ether,  alcohol  and  water 
and  then  replatinized  with  pure  platinum  chloride.  After 
cleaning  it  thoroughly  we  compared  the  electrodes. 

In  the  second  comparison  the  electrodes  became  constant 
in  potential  after  28  hours.  The  maximum  variation  between 
any  two  was  o .  1 1  millivolt.  Four  of  the  five  electrodes  were 
within  o .  02  millivolt  of  each  other. 

Twelve  new  sheet-electrodes  were  obtained  and  were  desig- 
nated by  the  numbers  8-19.  Nos.  8,  9,  12  and  13  were  platin- 
ized with  the  pure  platinum  chloride  used  for  the  first  elec- 
trodes; Nos.  14,  15,  1 6  and  17  were  platinized  with  ordinary 
platinum  chloride;  Nos.  10  and  n  were  platinized  first  with 
pure  platinum  chloride  and  then  with  the  ordinary  material, 
and  Nos.  18  and  19  were  left  bright. 

In  the  comparison  of  these  electrodes  another  form  of  ap- 
paratus1 was  used.  This  is  shown  in  Fig.  6.  In  the  actual 
comparison  only  one-half  of  the  apparatus  was  employed. 
When  the  second  half  was  used  it  contained  acid  of  another 
strength  for  the  comparison  of  the  potentials  of  the  electrodes 
in  acid  solutions  of  two  different  strengths.  The  bore  of  the 
ground  joint  (A)  which  joins  the  two  parts  of  the  apparatus 
was  made  of  the  same  size  as  that  used  in  the  pair  of  calomel 
cells  shown  in  Fig.  3,  so  that  the  hydrogen  electrodes  might 
be  measured  directly  against  a  calomel  electrode.  The  large 
tubes  (B)  had  an  inner  diameter  of  2.25  inches  and  were  6 
inches  deep.  This  size  enabled  us  to  compare  readily  8  or 
more  platinum  electrodes  (D)  with  each  other.  The  hydro- 
gen bubbled  in  through  the  small  side  tubes  (C),  and  escaped 
through  the  tube  (E),  which  was  bent  downward  at  the  top 
to  prevent  the  rapid  diffusion  of  air  back  into  the  cell. 

1  I  have  never  noticed  any  ill  effects  resulting  from  the  use  of  rubber  stoppers  in 
this  cell.  It  was  found  to  be  impossible  to  construct  a  glass  stopper  of  this  size  capa- 
ble of  holding  a  number  of  platinum  electrodes,  but  I  have  now  devised  another  type 
of  apparatus  in  which  the  rubber  stopper  is  absent.  Comparisons  will  show  whether 
the  rubber  stopper  is  objectionable.  I  can  now  compare  34  hydrogen  electrodes  at 
once. 


0 


ID 

c 

\E 

A 

3^^ 

cj 

1 

) 

_^ 

y 

B 

^ 

25 

Comparison  of  the  Electrodes. — The  new  electrodes,  Nos.  8- 
17,  were  not  electrolyzed  in  sulphuric  acid  before  comparison. 
They  were  merely  washed  thoroughly  with  water  and  then 
with  alcohol  and  ether  to  remove  any  grease  or  oil. 

In  the  first  experiment  Electrodes  Nos.  2,  3,  6,  12,  13,  16 
and  17  were  compared  with  each  other.  The  detailed  meas- 
urements are  given  in  the  following  table.  All  the  electrodes 
were  completely  immersed  in  o.i  N  hydrochloric  acid.  No. 
3  was  taken  as  the  comparison  electrode  and  considered  posi- 
tive and  the  readings  with  it  are  given  in  the  same  way  as  in 
the  comparison  of  the  calomel  electrodes.  The  readings  are 
expressed  in  hundred ths  of  a  millivolt: 

Time  26  12  13  16  17 

May  10,    10.30  A.  M.     Started 


May  n, 


The  potentials  of  six  of  the  seven  electrodes  show  a  max- 
imum variation  of  0.06  millivolt.  The  maximum  variation 
of  any  electrode  from  the  mean  of  all  the  electrodes  is  0.07 
millivolt;  the  average  variation  from  the  mean  is  0.03  milli- 
volt. 

In  the  second  experiment  electrodes  Nos.  i,  5,  8,  9,  10,  n, 
14  and  15  were  compared  with  each  other.  No.  5  is  taken  as 
the  comparison  electrode.  The  data  of  the  experiment  are 
given  in  the  following  table : 

May  12, 


May  13, 


I 

.00 

p. 

M. 

—8 

-637 

12 

—  18     — 

—  i 

2 

.00 

p. 

M. 

—4 

—272 

—  9 

—13     — 

—3 

4 

.00 

p. 

M. 

—6 

—  160 

10 

—13 

o 

5 

.00 

p. 

M. 

—6 

—109 

—  9 

10            ( 

>    —3 

9 

.00 

A. 

M. 

—  i 

—  17 

—  7 

—  7 

i 

10 

.00 

A. 

M. 

—  i 

—  10 

—  5 

—  5     — 

i 

Time 

1 

8 

9 

10 

11 

14 

15 

II 

50 

A. 

M. 

Started 

I 

•45 

P. 

M. 

—170 

—15 

21 

—14 

6 

—347 

—75 

2 

30 

P. 

M. 

—  98 

10 

—  14 

5 

6 

—153 

—  9 

3 

.40 

P. 

M. 

-36 

4 

—  4 

2 

3 

—  68 

—  3 

4 

.40 

P. 

M. 

—  28 

2 

2 

3 

3 

—  52 

o 

9.00 

P. 

M. 

—       2 

0 

0 

—  I 

0 

0 

0 

9 

30 

A. 

M. 

—     3 

o 

0 

2 

0 

—  17 

—  i 

12 

•30 

P. 

M. 

2 

2 

2 

0 

i 

2 

2 

3 

•45 

P. 

M. 

3 

3 

3 

3 

3 

3 

3 

26 

The  maximum  variation  between  any  two  electrodes  is 
0.06  millivolt.  The  maximum  variation  of  any  electrode 
from  the  mean  of  all  the  electrodes  is  0.05  millivolt,  and  the 
average  variation  from  the  mean  is  o .  02  millivolt. 

In  one  of  the  earlier  experiments  Nos.  3  and  5  were  found 
to  have  exactly  the  same  potential.  We  can  therefore  reduce 
all  the  results  of  the  two  above  tables  to  the  potentials  which 
should  be  given  when  electrode  No.  3  is  compared  against  any 
of  the  electrodes.  Considering  No.  3  positive,  we  obtain  the 
figures  i1 

1 2        5 6          8        9       10        11  12  13  14      15  16        17 

3—io    —io     333       3—5—5—33—1       i 

The  maximum  variation  of  any  electrode  from  the  mean 
is  0.095  millivolt.  The  mean  variation  from  the  mean  is 
0.030  millivolt.  I  shall  try  the  experiment  of  connecting 
different  electrodes  as  cells,  or  passing  a  current  through  them, 
to  see  if  the  potentials  can  be  made  more  nearly  equal. 

The  experiments  show  that  the  potential  of  the  hydrogen 
electrode  is  easily  reproduced  to  within  o.  io  millivolt  and  that 
the  potential  which  the  electrode  gives  is  independent  of  the 
purity  of  the  platinum  chloride  used  and  the  thickness  of  the 
coating  of  platinum  black,  above  a  certain  limit.  To  clean  the 
electrode,  it  need  not  be  used  as  cathode  in  the  electrolysis 
of  sulphuric  acid  nor  boiled  with  water;  rinsing  with  ether, 
alcohol  and  water  is  sufficient.  The  electrodes  may  be  com- 
pletely immersed  in  the  acid  solution  into  which  the  hydrogen 
gas  is  bubbled. 

4.    Comparison   of   the   Hydrogen    Electrode   with    the    Calomel 

Electrode 

Apparatus  and  Method  o)  Procedure. — The  apparatus  used 
in  the  comparison  of  the  hydrogen  electrode  with  the  calomel 
electrode  is  shown  in  Figs.  7  and  ya.  The  arrangement  of 
the  apparatus  there  is  that  which  was  employed  when  a  solu- 

1  In  this  calculation  it  is  assumed  that  the  relative  potentials  of  Nos.  3  and  5  are 
constant.  This  assumption  is  justified  by  later  experiments  in  which  Electrodes 
6  and  7  in  different  solutions  of  aniline  hydrochloride  were  found  to  maintain  con- 
stant relative  potentials.  I  shall  study  this  point  further. 


27 

tion  was  used  to  eliminate  the  contact  potential.  The  hydro- 
gen passed  from  the  palladium  asbestos  tube  (A)  through  the 
washing  apparatus  (B)  to  the  gauze  electrode  (G).  In  most 
of  the  experiments  a  sheet  electrode  (H)  was  also  used,  so 
that  one  electrode  would  serve  as  a  check  upon  the  other. 
The  solution  in  the  hydrogen-electrode  chamber  also  filled  the 
rest  of  the  piece  of  apparatus  (C).  The  end  of  (C)  dipped  into 
the  chamber  (D)  which  contained  the  saturated  solution  for 
eliminating  contact  potential.  This  solution  was  prevented 
from  diffusing  back  into  the  hydrogen-electrode  chamber  by 
the  two  stopcocks  on  (C).  In  the  chamber  (E)  was  placed 
a  o .  i  N  solution  of  potassium  chloride  the  same  as  that  in 
the  calomel  cell  (F).  This  solution  prevented  any  diffusion 
into  the  calomel  electrode  of  the  solution  for  eliminating 
contact  potential. 

In  the  earlier  experiments  the  hydrogen  electrode  was  com- 
pared directly  with  the  calomel  electrode  without  the  use  of 
any  solution  for  eliminating  contact  potential.  In  such  experi- 
ments chamber  (D)  served  as  the  hydrogen-electrode  chamber 
and  potassium  chloride  solution  was  put  in  (E).  The  piece  of 
apparatus  (C)  was  not  used.  The  stopcocks  were  always 
closed  when  measurements  were  not  being  made,  and  often 
even  during  measurements.  The  thin  film  of  solution  around 
the  stopper  served  to  conduct  the  current,  although  under 
these  conditions  the  measurements  were  not  quite  so  accurate. 
This  procedure  served  to  stop  diffusion  and  the  attendant 
changes  in  potential. 

After  some  preliminary  experiments  the  following  method  of 
procedure  was  adopted:  The  comparison  calomel  electrode 
was  first  compared  with  the  calomel  electrodes  of  the  battery 
and  then  placed  in  position  for  use  with  the  hydrogen  elec- 
trode. The  objection  might  be  raised  to  this  method  of  pro- 
cedure, especially  in  view  of  the  experience  of  Coggeshall, 
that  the  comparison  electrode  would  change  in  potential 
by  being  moved  around.  To  eliminate  any  such  source  of  error 
the  comparison  calomel  cell  was  again  compared  with  the 
battery  at  the  completion  of  the  experiment.  This  was 
hardly  necessary,  however,  as  the  following  experiment  to 


28 

test  the  effect  of  mechanical  disturbance  shows.  Cell  No.  14, 
one  of  the  comparison  electrodes,  gave  a  voltage  against 
No.  7  of  the  battery  of  0.00029  volt.  No.  14  was  then  moved 
around  and  put  back  again  with  the  battery.  The  voltage 
of  14  17  was  0.00030,  a  change  of  only  o.ooooi  volt.  A  still 
more  striking  proof  of  the  small  effect  of  mechanical  disturb- 
ance occurred  by  accident.  On  February  3,  14  :  7  gave  a 
voltage  of  0.00023.  On  the  morning  of  February  4  No.  14 
fell  over  in  the  oil  bath,  flat  on  its  side.  It  was  quickly  picked 
up,  the  side  tube  below  the  stopcock  freed  from  oil  (the  stop- 
cock was  closed  at  the  time  of  the  accident)  and  immediately 
compared  with  No.  7 ;  14  :  7  gave  a  reading  of  o .  00030,  a  change 
of  only  o .  00007  volt  being  caused  by  the  accident. 

The  figures  just  given  show  the  reason  for  the  method  of 
procedure  adopted.  Whereas  the  calomel  electrodes  in  the 
battery  were  very  nearly  constant  in  value,  the  potential  of 
the  comparison  electrode  fluctuated  from  day  to  day,  being 
generally  in  the  neighborhood  of  two  to  three-tenths  of  a  milli- 
volt lower  in  potential  than  the  cells  of  the  battery.  This  is 
to  be  explained  by  the  constant  disturbance  this  cell  was  sub- 
jected to  and  also  to  the  likelihood  of  impurities  diffusing  into 
the  cell  during  measurements. 

Measurement  of  the  Hydrogen  Electrode  against  the  Calomel 
Electrode, — A  typical  experiment  in  which  the  hydrogen  elec- 
trode was  compared  with  the  calomel  electrode  is  given  below. 
The  calomel  electrode  is  positive. 

An  experiment  in  which  two  electrodes  were  used,  covering 
considerably  more  time,  is  given  on  page  29.  The  gauze  elec- 
trode, it  will  be  noted,  reaches  a  constant  potential  much 
sooner  than  the  sheet  electrode.  "No.  14"  is  the  calomel 
electrode,  "5"  and  "gauze"  are  the  two  hydrogen  electrodes. 

The  figures  given  in  the  seventh  column  should  theoretically 
be  the  difference  between  those  in  the  third  and  fifth  columns. 
The  comparison  of  the  observed  and  calculated  differences 
shows  the  accuracy  of  the  measurements. 


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10  :  14  =  0.00030 


Time  E.  M.  F. 

10.30  A.  M.  Started 

20  O . 4050 

24  0.4153 
56  0.42547 

II. 12  0.42590 

38  0.42610 

12.03  P.  M.  0.42616 

25  0.42619 
31  0.42619 

37  0.42619 

Stopped 

Observed  K.  M.  F.  0.42619 

Correction  for  calomel  cell  +  o .  00030 
Barometric  pressure, 1  o .  983  ats. ; 

bar.  correction  +0.00044 


of 


E.M.  F. 
corrected 
for  bar. 

0.42624 


E.  M.  F.  0.42693 

The  following   table  gives   the   summary  of  a  number 
experiments  made  in  this  way : 

E.  M.  F.  cor- 
rected for 

Pt  electrode  Bar.  pres.  E.  M.  F,  calomel 

used  in  ats.  observed  electrode 

"Gauze"    1.014    0.4265  0.4266 

No.  5      1.014    0.4261  0.4262 

No.  3      i. ooo    0.4265  0.4266  0.42660 

No.  2      i. ooo    0.4263  0.4264  0.42640 

No.  4      1.003    0.4266  0.4267  0.42662 

No.  i       1.003    0.4262  0.5267  0.42662 

No.  6      0-983    0.4262  0.4265  0.42694 

No.  6       1.007    0.4266  0.4264  0.42622 

"Gauze"    1.009    0.4272  0.4267  0.42647 

"Gauze"    1.002    0.4269  0.4266  0.4265 

No.  5      0.998    0.4269  0.4266  0.42665 

"Gauze"    0.998    0.4270  0.4267  0.42675 

"Gauze"    0.998    0.4269  0.4266  0.42665 

No.  5      0.998    0.4268  0.4265  0.42655 


Average  0.42656   0.42652 

1  The  barometric  pressure  is  given  in  atmospheres;  i.  e.,  760  mm.  at  0°  C.  and 
45°  latitude.  This  does  not  include  a  correction  of  about  23.5  mm.  for  the  vapor 
tension  of  the  solutions  at  25°.  This  correction  seems  never  to  have  been  made  by 
others.  The  error  involved  when  barometer  readings  are  referred  back  to  760  mm. 
partial  pressure  of  hydrogen,  as  shown  by  the  data  presented  in  this  article,  is  only 
about  0.00001  volt  for  the  ordinary  pressures. 


If  the  very  low  value  0.42584  is  omitted  the  average  is 
0.42657,  whether  the  barometer  correction  is  applied  or  not. 
If  the  value  0.42584  is  omitted,  the  mean  deviation  of  the  in- 
dividual readings  from  the  average  is  o.ooon,  and  the  max- 
imum deviation  is  0.00037.  As  a  working  average  I  shall 
use  0.4266.  This  is  very  close  to  the  value  0.4270  found  by 
Bjerrum. 

Besides  the  experiments  given  above,  there  were  also  a  few 
experiments  in  which  values  were  obtained  not  agreeing  with 
the  others,  but  in  which  it  was  shown  that  the  platinum  elec- 
trode was  at  fault. 

H2  —  PtNo  l  —  o.  i  N  HC1  —  o.  i  N  KC1  —  No.  14 
10  :  14  =  0.00050 

Time  E.  M.  F. 

i .  oo  P.  M.  Started 

2.24  0.3666 

2.36P.  M.  0.3675 

Electrode  moved 

2.38  P.  M.  0.3666 

2.41  P.  M.  0-3674 

2  .44  P.  M.  0.3682 

2  .  48  P.  M.  O  .  368 

2.55P.M.  0.3694 

Electrode  turned  around 
3.06P.M.  0.3659 

Experiment  stopped 

On  inspection  the  coating  of  platinum  black  was  found 
to  be  very  thin.  The  electrode  was  replatinized  with  pure 
platinum  chloride  and  electrolyzed  in  sulphuric  acid. 

We  then  repeated  the  above  experiment. 

H2  —  PtNo. !  —  o.  i  N  HC1  —  o.  i  N  KG  —  No.  14 
10  :  14  =  0.00047.     Bar.  pres.,  1.003  ats. 

Time  E.  M.  F.  Time  E.  M.  E. 

9-ooA.M.  Started        II.OIA.  M.  0.42620 

10.08  A.  M.         0.42524    II.lSA.  M.         0.42630 
10.  13  A.  M          0.42567    II. 40  A.M.         0.42615 

10.23  A.  M.  0.42604          0.42615  =  obs.  E.  M.  F. 

10. 29  A.  M.  0.42615  +0.00047  =  corr.  for  No.  14 

10.44  A-  M-  0.42619  — 0.00008  =  corr.  for  bar. 

10.52  A.  M.  0.42620          0.42654  =  corr.  E.  M.  F. 


32 

It  is  seen  that  the  value  0.42654  now  given  by  Electrode 
No.  i  is  in  good  agreement  with  the  average  value  of  a  large 
number  of  experiments  given  above. 

In  the  experimental  work  dealing  with  the  comparison  of 
the  hydrogen  electrodes  Electrode  4  was  broken.  In  resealing 
the  platinum  wire  into  another  piece  of  glass  the  platinum 
black  was  turned  to  gray  by  ignition.  This  gray  electrode 
was  tried  in  one  experiment : 

H2  — PtNo  4  — o.i  N  HC1  — o.i  N  KC1  — No.  14 

10  :  14  =  0.00014 
Calomel  electrode  considered  positive 

Time  E.  M.  F. 

9.00  A.  M.  Started 

10. 12  P.    M  0.00344 

10.  l6  A.  M.  0.00280 

10.23  A-  M-  0.00240 

10.48  A.  M.  0.00336 

Stopped 

Electrode  4  was  now  platinized  with  ordinary  platinum 
chloride  solution,  electrolyzed  in  sulphuric  acid,  washed  with 
water  and  again  used  to  check  the  above  experiment.  This 
time  a  value  of  0.4267  was  obtained. 

Discussion  of  Results. — In  the  value  0.42657  of  the  poten- 
tial difference  measured  in  these  experiments  are  included 
three  factors,  the  potential  of  the  calomel  electrode,  the  poten- 
tial of  the  hydrogen  electrode  and  the  contact  potential  of 
the  two  solutions,  decinormal  hydrochloric  acid  and  deci- 
normal  potassium  chloride.  For  calculating  the  contact  poten- 
tial of  two  solutions  various  formulas  have  been  proposed. 
The  first  formula  was  that  of  Planck.1  When  applied  to  two 
solutions  having  the  same  concentration  this  formula  becomes 

u.  +  v2 
*2s*  =  0.059  Iog10  2pr^ 

where  uv  u2t  ivl  and  v2  represent  the  migration  velocities  of 
the  anions  and  cations  of  the  two  solutions.     Using  the  data 
of  Kohlrausch  and  Holborn  we  obtain  at  25°  the  values 
H+  =  352.1,  K+  =  74.5,  and  Oh  =  75-3 

1  Wied.  Ann.,  40,  561  (1891). 


33 

Substituting  these  data  in  the  equation  we  obtain  0.0274 
as  the  value  of  the  contact  potential  at  25  °. 

There  have  been  various  modifications  of  this  formula,  for 
example,  that  of  Henderson1  and  that  of  Lewis  and  Sargent.2 
Bjerrum,3  in  a  discussion  of  the  accuracy  of  Planck's  and  Hen- 
derson's formulas,  gives  0.0277  as  the  value  of  the  contact 
potential  between  o  .  i  N  potassium  chloride  and  o  .  i  N  hydro- 
chloric acid.  He  also  noticed  that  in  the  case  of  some  solu- 
tions there  was  a  small  change  in  the  potential  at  first,  due  to 
diffusion  at  the  planes  of  contact  between  the  solutions. 

Lewis  and  Sargent's  formula  has  the  form 


.         , 

*  ~  T  log'  t 

where  ^  and  X2  represent  the  equivalent  conductivities  of  the 
two  solutions.  Lewis  and  Rupert4  give  the  values  389.9  for 
the  equivalent  conductivity  of  o  .  i  N  hydrochloric  acid  and 
128.8  for  o.i  N  potassium  chloride.  These  data  substituted 
in  the  above  equation  give  0.0284.  The  same  data  when  ap- 
plied to  Planck's  original  formula  give  0.0266.  In  the  arti- 
cle by  Lewis  and  Sargent  o  .  0286  is  the  value  given  and  further 
evidence  for  this  value  is  found  in  the  fact  that  it  is  identically 
the  same  as  the  value  found  by  Sauer  for  the  potential  differ- 
ence of  the  combination 

Hg  —  HgCl  —  o.i  NKC1  —  o.i  NHC1  —  HgCl  —  Hg 

Lewis  claims  to  have  ample  proof  that  o  .  i  N  potassium  chloride 
and  o  .  i  N  hydrochloric  acid  are  equally  dissociated  (86  per 
cent.).  If  the  concentration  of  chlorine  ions  is  the  same  in 
each  solution,  of  course  the  potential  of  the  o  .  i  N  potassium 
chloride-calomel  electrode  will  be  identical  with  the  potential 
of  the  o  .  i  N  hydrochloric  acid-calomel  electrode,  and  the  whole 

1  Z.  physik.  Chem.,  59,  118  (1907);  63,  325  (1908). 

2  J.  Am.  Chem  Soc.,  31,  363  (1909). 

3  Z.  Elektrochem.,  17,  58  (1911). 

4  J.  Am.  Chem.  Soc.,  33,  306  (1911). 


34 


difference  found  in  comparing  the  two  electrodes  will  be  due 
to  the  contact  potential. 

We  thus  see  that  by  the  use  of  different  data  and  formulas 
the  calculated  values  of  the  contact  potential  between  o .  i  N 
hydrochloric  acid  and  o .  i  N  potassium  chloride  vary  from 
0.0266  to  0.0286.  As  this  difference  is  much  greater  than  is 
desirable,  it  was  attempted  to  use  some  solution  to  eliminate 
the  contact  potential. 

Experiments  to  Determine  the  Efficiency  of  Several  Solutions 
for  Eliminating  Contact  Potential. — Following  the  suggestion 
of  Abegg  and  Cumming,  ammonium  nitrate  was  first  used. 
In  these  experiments  the  arrangement  of  apparatus  was  that 
shown  in  Fig.  7. 

The  summary  of  three  experiments  with  the  combination 

H2  —  Ft  —  o.  i  N  HC1  —  saturated  NH4NO3  —  o.  i  N  KC1  — 

HgCl  — Hg 

is  as  follows : 

E.  M.  F. 


Experiment 

I 
2 
3 


Bar.  pres. 
in  ats. 

0.908 
I  .009 
I  .002 


Corrected  for 
calomel  cell 

Corrected  for 
bar.  pres. 

0.3990 
0-3998 

0-3995 

0-39931 

0-39957 
0.39946 

Ave  age  value      o .  3994 


0-39944 


The  presence  of  the  saturated  ammonium  nitrate  solution 
causes  a  difference  in  electromotive  force  of  o. 4266 — 0.3994  = 
0.0272. 

The  requisites  of  a  good  salt  for  eliminating  contact  poten- 
tial are  that  it  shall  be  very  soluble  and  that  the  velocities 
of  the  ions  shall  be  nearly  equal.  Potassium  chloride,  while 
not  nearly  as  soluble  as  ammonium  nitrate,  has  almost  iden- 
tical velocities  for  its  ions.  Saturated  ammonium  nitrate 
is  about  1 1  N ;  saturated  potassium  chloride  at  25  °,  4 . 1 2  N. 
The  effect  of  4 . 1 2  N  potassium  chloride  solution  was  next 
tried.  The  following  table  is  a  summary  of  the  experiments. 
Two  electrodes  were  used  in  each  case : 


Corrected 

Corrected  for 

calomel  cell 

bar.  pres. 

O.4002 

o  .  40005 

o  .  4002 

o  .  40005 

0.40OO 

o  .  40000 

0.4000 

O.4OOOO 

35 

H2  —  Pt  -  -  o.  i  N  HC1  —  saturated  KC1  —  o.  i  N  KC1 

HgCl  — Hg 

E.  M.  F. 

Bar.  pres. 
Experiment  in  ats. 

1  I . 006 

2  I . OOO 

Average  value      o .  4001  o .  4000 

The  saturated  potassium  chloride  solution  causes  a  differ- 
ence of  o .  4266  —  o .  4000  =  o .  0266  volt. 

It  was  impossible  to  tell  from  these  experiments  whether 
it  was  better  to  use  potassium  chloride  or  ammonium  nitrate, 
so  a  series  of  experiments  was  run  with  different  concentra- 
tions of  acids.  Here  it  was  possible  to  calculate  the  theo- 
retical difference  in  the  electromotive  force  due  to  the  known 
change  in  hydrogen  ion  concentration.  By  comparing  this 
theoretical  difference  with  the  observed  difference,  the  rela- 
tive efficiency  of  different  solutions  for  eliminating  contact 
potential  could  be  determined.  Besides  ammonium  nitrate 
and  potassium  chloride,  the  effect  of  potassium  iodide,  potas- 
sium bromide  and  calcium  acetate  was  determined. 

In  calculating  the  hydrogen  ion  concentration  of  the  hydro- 
chloric acid  solutions  used  the  following  dissociation  values 
were  taken : 

N  HC1      8 1 .  o  per  cent.     Determined  by  Sauer 
.o.  i  N  HC1      92 . 2  per  cent.  ] 

o.oi  N  HC1      96 . 9  per  cent.  |  Determined  by  A.  A.  Noyes1 

o .  ooi  N  HC1  100 .  o  per  cent,  j 

The  following  table  summarizes  the  results: 

1  The  Electrical  Conductivity  of  Aqueous  Solutions,  Carnegie  Institution  Publi- 
cation No.  63,  page  141. 


00 


W  00  00 

CO   d  NO 

SS  o 

do  6 


CN 
00 


2 


oo 
r- 


oo 

5" 


ON  r-.       ON  ci 

t>-  r>.       r>»  oo 

10  *o       10  "^ 

O  O         O  O 

odd  d 


Tj-OO 
\D  VO 
PO  CO 


CO  CO 


:<§<£ 


Svg 


to      ^  -^      »o  »o  co  co 

d      do      d  d  d  d 


o  vo 

vO  vo 

d  d 


«O  VO  OO    O 
M    C4    H«    N 


»O  »O  CO  CO 

d  o*  o*  6 


T*- 
d 


00  00 
OO  00 

t^  t^ 

U-)    IO  MM 

Ti-  T*-       10  10 

do      do 


M         00  00 


o 


»0  10 

o'o 


1 


1     1 


d    § 


CO 


8       8 


IO        O\  vO 

8^8 


3 


37 


vO 

00 

00 

w 

l>. 

r^. 

t^ 

00 

u 

to 

to 

to 

to 

"3 

0 

O 

0 

0 

O 

1 

d 

d 

d 

d 

s 

«a 

5 

d 

Tf 

M 

ftO 

5 
O 

M 
l>». 

O 

VO 

» 

CO 

* 

1 

d 

d 

d 

d  d 

00 


VO     M 

o  o 
^o  ^o 
o  o 

d  d 


oo 

o1 


' 


1 


S« 

SI 

JJ 


to  to  »o  to 

M      M      CO    CO 


fO  CO 


88     22 


CO  CO  rt-  rj- 

d  d  d  d 


vO  vO   t***  I** 

vO  vO    CO  co 
CO  CO  ^-  ^ 

d  d  d  d 


to  to  co  co 

d  o"  d  d 


to  to  O  O 


to  tO  CO  CO 

d  d  d  d 


o  o 

M     M 

to  to 

d  6 


M     M 

to  to 

Tj-   Tt- 

d  d 


t>»vO    to  to    M  vO 
l^*  t*1^   \&  vO    t^**  vO 

tO  tO    CO  CO    Tj-  Tf 


O  O 


O   O 


0   0 


o  o      o  o 


OO  OO    OO  CO   vO  OO 

MM  ONO\         tOto         MM 

tOto         COCO         <i*,  ^"         totO 


ON 

d  d 


o  o      o  o 


^1^8          1  :«          t      I 
252S  82  58 


a  M 


M?! 


00 

§ 


o      M 


r>» 

d 


« 

M 


The  results  given  in  the  above  table  indicate  that  potassium 
chloride  is  by  far  the  most  efficient  of  the  salts  used  for  elimina- 
ting contact  potential.  This  conclusion  is  confirmed  by  some 
results  obtained  with  the  decinormal  hydrochloric  acid-calo- 
mel electrode. 

Potential  of  H2—Pt—o .  i  N  HCl—o .  i  N  HCl—HgCl—Hg.— 
Two  decinormal  hydrochloric  acid-calomel  electrodes  were 
prepared  and  measured  against  the  hydrogen  electrode  in  deci- 
normal hydrochloric  acid  solution.  The  following  corrected 
values  were  obtained  for  the  electromotive  force  of  the  series 
H2  —  Pt  —  o.  i  N  HC1  —  o.  i  N  HC1  —  HgCl  —  Hg: 

Bar.  pres.  E.  M.  F.  corr. 

in  ats.  E.  M.  F.  for  bar.  press. 

0.994  0-3999  0.40005 

o .  999  o . 4003  o . 40006 

i.oio  0.4005  0.40024 

Average    o .  4002  o .  400 1 2 

If  we  accept  Lewis's  conclusions  that  the  potential  of  the 
o.i  N  HC1  —  HgCl  —  Hg  electrode  is  the  same  as  that  of  the 
o.i  N  KC1  — HgCl  — Hg  electrode,  then  the  difference  be- 
tween the  electromotive  force  of  the  element 

H2  —  Pt  —  o.  i  N  HC1  —  o.  i  N  KC1  —  HgCl  —  Hg 
and  that  of  the  element 

H2  —  Pt  —  o.  i  N  HC1  —  o.  i  N  HC1  —  HgCl  —  Hg 

should  be  equal  to  the  contact  potential  of  the  system  o .  i  N 
HC1  — o.iNKCl.  The  actual  difference  which  I  find  is 
0.4266  —  0.4001  =  0.0265.  This  value  is  very  close  to  the 
difference  which  I  find  between  the  electromotive  force  of 
the  element 

H2  —  Pt  —  o.  i  N  HC1  —  o.  i  N  KC1  —  HgCl  —  Hg 
and  that  of  the  element 

H2  —  Pt   -  -  o.  i  N  HC1  —  saturated  KC1  —  o.  i  N  KC1  — 

HgCl  —  Hg 

viz.,  0.4266  —  0.4000  =  0.0266,  the  change  in  potential  due 
to  the  presence  of  concentrated  potassium  chloride  solution. 

If,  therefore,  the  assumption  of  Lewis  in  regard  to  the  iden- 


39 

tity  in  potential  of  the  hydrochloric  acid  and  potassium  chlor- 
ide electrodes  is  granted,  we  reach  the  conclusion  from  the 
experiments  with  the  hydrochloric  acid  electrode,  as  well  as 
from  those  in  which  potassium  chloride  was  used  to  eliminate 
contact  potential,  that  a  saturated  solution  of  potassium  chlor- 
ide eliminates  almost  completely  the  contact  potential  of  liquid 
systems,  at  least  those  composed  of  potassium  chloride-hydro- 
chloric acid. 

It  will  be  noticed,  however,  that  the  acceptance  of  the  above 
assumption  of  Lewis  also  involves  the  acceptance  of  the  value 
of  86  per  cent,  for  the  dissociation  of  o.  i  N  hydrochloric  acid. 
If  this  value  is  used  the  agreement  of  the  figures  obtained  for 
the  efficiency  of  potassium  chloride  for  eliminating  contact 
potential  is  by  no  means  so  good  unless  a  corresponding  de- 
crease in  the  dissociation  of  the  other  concentrations  of  hydro- 
chloric acid  is  assumed.  Inasmuch  as  the  value  of  92.2  per 
cent,  for  the  dissociation  of  o .  i  N  hydrochloric  acid  is  the 
value  generally  accepted,  this  is  the  value  which  has  been 
used  in  calculating  the  potential  of  the  calomel  electrode, 
but  the  other  values  are  also  discussed. 

It  should  be  stated  that  the  values  obtained  in  the  experi- 
ments involving  the  hydrochloric  acid-calomel  electrode  are 
by  no  means  as  certain  as  those  with  the  potassium  chloride- 
calomel  electrode,  for  the  reason  that  only  two  acid  electrodes 
were  prepared.  Work  is  now  in  progress  on  the  HCl-HgCl- 
Hg  and  the  H2SO4-Hg2SO4-Hg  electrodes. 

Potential  of  the  Decinormal  Calomel  Electrode 

On  the  assumption  that  a  concentrated  solution  of  potas- 
sium chloride  entirely  eliminates  the  contact  potential  of  the 
system  o.  i  N  HC1  —  o.  i  N  KC1,  the  value  of  the  electrode 
o.  i  N  KCl-HgCl-Hg  becomes  0.4000-0.0591  [ — log  (o.  1006  X 
0.922)]  =0.3390.  If  the  dissociation  of  o.i  N  hydrochloric 
acid  is  taken  as  86  per  cent.,  the  corresponding  value  of  the 
electrode  becomes  0.3372.  The  value  obtained  by  Sauer  is 
0.3406.  If  the  contact  potential  of  o.  i  N  HC1  —  o.  i  N  KC1 
is  given  the  value  0.0284  assigned  by  Lewis,  or  0.0286  found 
by  Sauer,  experimentally,  and  86  per  cent,  is  taken  as  the 


40 

degree  of  dissociation  of  o .  i  N  hydrochloric  acid,  the  value 
of  the  calomel  electrode  becomes  0.3355.  ^  *s  evident  that 
we  cannot  draw  final  conclusions  until  we  know  more  accurately 
the  per  cent,  of  ionization  of  all  the  electrolytes  concerned  and 
have  a  very  accurate  method  to  calculate  contact  potential, 
and  an  experimental  method  to  eliminate  it  completely.  Such 
measurements  of  the  electromotive  force  of  various  systems 
will  perhaps  be  very  helpful  in  this  direction. 

In  the  experiments  in  the  following  article  the  value 
o .  339  is  used  as  the  potential  of  the  system  o .  i  N  KC1  — 
HgCl  —  Hg,  and  the  value  o .  3355  is  compared  with  this  in 
the  discussion. 

SUMMARY 

This  series  of  experiments  has  shown  that: 

1.  Calomel  electrodes,  o.  i  N  KC1  —  HgCl  —  Hg,  can  be  pre- 
pared which  for  the  first  three  weeks  vary  not  more  than  o.  10 
millivolt.     With    longer    standing    the    variation    slowly    in- 
creases. 

2.  Platinum  electrodes  can  be  prepared  which,  when  used 
as  hydrogen  electrodes  in  o.  i  N  hydrochloric  acid,  show  a  varia- 
tion from  the  mean  value  of  less  than  o.  10  millivolt. 

3.  The  electromotive  force  of  the  system 

H2  —  Pt  —  o.  i  N  HC1  —  o.  i  N  KC1  —  HgCl  — Hg 

is  0.4266. 

4.  Saturated  potassium  chloride  solution  eliminates  almost 
completely   the   contact   potential   of   systems   consisting   of 
potassium  chloride  and  hydrochloric  acid. 

5.  The  value  of  the  potential  of  the  electrode  o.  i  N  KC1 — 
HgCl — Hg  is  o .  339  if  the  dissociation  of  o .  i  N  hydrochloric 
acid  is  92 . 2  per  cent. ;  it  is  o. 337  if  the  dissociation  of  the  acid 
is  86  per  cent.,  and  0.3355  if  the  contact  potential  is  assumed 
to  be  0.0284.     The  results  obtained  by  using  these  different 
values  are  discussed  hi  the  following  article  and  are  found  to 
harmonize  better  with  Lewis's  data. 


The  Application  of  the  Hydrogen  Electrode 

to  the  Measurement  of  the  Hydrolysis 

of    Aniline  Hydrochloride,  and  the 

lonization  of  Acetic  Acid  in  the 

Presence  of  Neutral  Salts 


We  organic  chemists  have  been  nearly  completely  baffled 
in  the  study  of  some  of  our  reactions  because  of  the  lack  of 
some  direct,  accurate  and  very  rapid  method  for  determining 
the  concentration  of  hydrogen  ions  (also  hydroxyl,  chloride, 
bromide,  sulphide  ions)  in  the  presence  of  all  organic  com- 
pounds, especially  when  the  system  is  undergoing  change.1 
We  have  methods  involving  conductivity,  catalysis,  dilata- 
tion, colorimetry,  etc.,  some  or  all  of  which  can  be  applied 
reasonably  well  in  some  cases;  but  all  of  these  methods  may 
fail  utterly  in  special  cases,  especially  when  the  solution  is  con- 
stantly varying  in  composition. 

When  acetamide  (or  any  ester,  oxime,  etc.)  is  hydrolyzed 
in  the  presence  of  hydrochloric  acid  a  small  amount  of  the 
salt  of  the  amide  is  formed,  and  the  concentration  of  this  salt 
is  constantly  diminished  as  the  amide  disappears. 

CH3CONH2  +  HC1  +  H  +  Cl  +  CH3CONH3C1  4-  CH3CONH3  + 
H2O  — >     CH3COOH  +  NH4  +  Cl,  etc. 

We  have  no  method  to-day  for  determining  the  concentra- 
tions of  the  constituents  of  such  a  system,  as  there  are  too 
many  unknowns  in  the  equation.  If,  however,  we  had  a  di- 
rect, accurate,  instantaneous  method  for  determining  at  any 
moment  the  concentration  of  the  hydrogen  ions,  we  could 
calculate  the  concentration  of  the  amide  salt  and  its  ions, 
and  could  then  determine  directly  whether  this  amide  salt, 
or  its  ions,  or  some  other  constituent,  is  the  substance  directly 

1  See  the  address  of  the  Chairman  of  the  Division  of  Organic  Chemistry  in  Section 
C  of  the  American  Association  for  the  Advancement  of  Science,  Baltimore,  1908. 
Science,  30,  624. 


42 

yielding  the  end  products.  We  have  the  same  case  in  the  de- 
composition of  amides,  esters,  etc.,  by  alkalis,  and  just  as 
great  a  need  for  a  satisfactory  method  for  determining  the  con- 
centration of  hydroxyl  ions. 

With  these  facts  in  mind,  and  with  the  advantage  of  the 
experiences  of  Desha1  in  this  investigation,  I  have  taken 
up  again  the  attempt  to  apply  the  hydrogen  electrode  to  this 
problem.  In  the  present  communication  I  am  presenting 
some  experiments  bearing  on  the  accuracy  of  the  hydrogen 
electrode  for  determining  the  concentration  of  hydrogen  ions 
in  the  presence  of  the  organic  substances  aniline  and  acetic 
acid.  I  have  chosen  these  simple  compounds  because  the 
substances  are  stable,  the  constants  which  I  wish  to  measure 
have  been  accurately  determined  by  other  methods,  and  I 
eliminate  the  uncertainties  due  to  the  changes  in  a  reacting 
system.  The  question  of  the  rapidity  attainable  in  measuring 
the  concentration  of  the  hydrogen  ions  of  a  solution  has  been 
considered  in  an  article  by  Desha  and  Acree.  So  many 
more  unforeseen  difficulties  have  beset  me  in  this  work  with 
organic  compounds  than  ever  occur  in  work  with  inorganic 
substances  that  I  shall  present  these  difficulties  rather  fully 
for  the  benefit  of  others. 

Hydrolysis  of  Aniline  Hydrochloride 

One  of  my  first  experiments  to  learn  whether  the  hydrogen 
electrode  might  be  applied  to  organic  reactions  was  the  de- 
termination of  the  hydrolysis  of  aniline  hydrochloride. 

The  hydrolysis  of  this  salt  had  been  carefully  determined 
by  Bredig2  by  the  conductivity  method  to  be  2 . 63  per  cent, 
at  25°  in  N/32  solution.  Denham  had  applied  the  hydrogen 
electrode  to  this  problem  and  determined  the  hydrolysis  of 
aniline  hydrochloride  in  N/i6,  N/24  and  N/32  solutions. 
For  the  N/32  solution  he  obtained  the  value  2.58  per  cent., 
a  result  agreeing  remarkably  well  with  the  value  determined 
by  Bredig  for  the  same  solution.  There  are  two  small  points 
in  Denham 's  work,  however,  in  which  there  is  a  chance  for 
difference  of  opinion.  In  the  first  place  the  number  2.58  is 

1  Desha:  Diss.,  Johns  Hopkins  Univ..  1909. 

2  Z.  physik.  Chem..  13,  289  (1894). 


43 

not  the  percentage  hydrolysis  of  the  N/32  solution  of  aniline 

hydrochloride,  but  rather  the  ratio  1H' conc-l  *  IO°.     To ob- 

[total  salt} 

tain  the  degree  of  hydrolysis  this  value  must  be  divided  by 
0.96,  the  degree  of  dissociation  of  N/32  hydrochloric  acid. 
This  raises  the  degree  of  hydrolysis  to  2.69.  The  second 
point  is  a  much  more  vital  one.  He  uses  the  number  o .  56  as  the 
value  of  his  normal  calomel  electrode.  This  value  is  the  one 
determined  by  Rothmund  by  the  drop-electrode  method  at 
1 8°.  Denham's  measurements  were  carried  out  at  25°.  If 
we  apply  the  temperature  factor  of  the  normal  calomel  elec- 
trode as  determined  by  Richards1  we  obtain  for  the  potential 
of  the  electrode  at  25°  the  value  0.56  +  (7  X  0.0006)  = 
0.564.  If  this  value  is  used  in  the  calculations  instead  of 

\H'  cone}   X  TOO    -  x , ,  A. 

0.56,   tne   ratio   * F ^ — r-^ for   the   N/32   solution   is 

[total  salt] 

found  to  be  3.02  instead  of  2.58,  and  the  per  cent,  of  hy- 
drolysis becomes  3.15,  a  value  differing  quite  widely  from  that 
obtained  by  Bredig. 

Desha  attempted  to  repeat  the  experiments  of  Denham, 
along  with  other  experiments  of  his  own,  in  this  laboratory, 
but  had  little  success.  For  the  N/32  solution  Desha  found 

,.     \H'  cone}   X  100  ,  i  i    *    1  •  a 

the  ratio  i — ? r^ — ^ to  be  5  .  79.     He  was  troubled  chiefly 

[total  salt] 

by  the  decomposition  of  his  material,  the  solution  acquiring 
a  pink  color  after  the  experiment  had  proceeded  for  a  time. 

The  aniline  hydrochloride  prepared  by  Desha  had  not  been 
recrystallized,  whereas  the  material  used  by  Denham  was  re- 
peatedly recrystallized  from  acetone  and  finally  washed  with 
ether.  Thinking  that  the  cause  of  the  discrepancies  between 
the  results  of  Denham  and  Desha  might  be  due  to  impurities 
present  in  Desha's  salt,  I  prepared  my  aniline  hydrochloride 
with  considerable  care,  especially  as  Dr.  Denham  had  kindly 
told  me  of  his  own  difficulties  in  this  connection. 

The  aniline  was  fractionally  distilled  twice,  the  fraction 
boiling  between  182°  and  183°  being  used  for  the  preparation 
of  the  hydrochloride.  It  was  dissolved  in  ether  and  the  hy- 
drochloride precipitated  by  passing  in  dry  hydrochloric  acid 

i  Z.  physik.  Chem.,  24,  53  (1897). 


44 

gas,  the  solution  being  kept  cold  by  an  ice  bath.  The  white 
crystals  were  filtered  off,  washed  repeatedly  with  ether  and 
dried  over  sulphuric  acid  and  caustic  potash  in  vacua.  This 
formed  Sample  I. 

Of  this  dry  salt  4.0478  grams  were  dissolved  in  500  cc.  of 
conductivity  water  to  form  a  N/i6  solution.  A  portion  of 
this  was  diluted  to  form  a  N/32  solution. 

The  results  obtained  with  these  first  two  solutions  are  shown 
in  the  following  table.  In  accordance  with  the  practice  of 
both  Denham  and  Desha,  saturated  ammonium  nitrate  solu- 
tion was  used  to  eliminate  the  contact  potential.  The  value 
of  the  calomel  electrode  used  is  o  .  339. 

CoS^tedfor  V*' 


Concentration  calomel  electrode  [total  salt] 

N/i6  0.5066  2.30 

0.5003  2.95 

N/32  0.5124  3.68 

0.4943  7.47 

N/32  0.5072  4.51 

o  .  5064  4  .  64 

N/32  0.5129  3.61 

N/32  Indefinite 

As  will  be  noted,  the  results  are  extremely  discordant. 
Almost  invariably  the  gauze  electrode  gave  a  higher  value 
than  the  sheet  electrode.  To  see  if  the  electrodes  were  at 
fault,  they  were  tested,  after  the  first  experiment,  in  a  solu- 
tion of  o  .  i  N  hydrochloric  acid,  the  electrodes  having  first 
been  washed  with  alcohol  and  ether.  Both  electrodes  gave 
the  same  potential,  this  fact  showing  that  they  were  all  right. 
It  was  noted  in  each  of  these  experiments,  after  the  removal 
of  the  electrodes,  that  there  was  oil  on  the  surface  of  the  solu- 
tion. As  no  trouble  of  this  kind  was  experienced  with  solu- 
tions of  acids,  it  is  probable  that  the  oil  was  present  at  first 
in  the  aniline  hydrochloride,  or  was  the  product  of  the  de- 
composition of  some  substance  present  in  the  aniline  hydro- 
chloride  solution.  As  in  these  experiments  it  had  been  the 
custom  to  have  the  sheet  electrode  partially  out  of  the  solu- 
tion, the  lower  value  of  this  electrode  is  probably  due  to  its 
becoming  coated  with  this  oil.  In  subsequent  experiments 


45 


the  sheet  electrode  was  entirely  immersed,  but  in  the  other 
samples  of  aniline  hydrochloride,  which  were  further  purified 
as  described  below,  no  traces  of  oil  were  found.  No  pink 
color,  described  by  Desha,  was  noticed  in  the  solution  in  any 
of  the  experiments.  Constant  readings  were  generally  ob- 
tained in  three  hours,  the  drift  being  very  small  after  the  first 
hour  and  a  half.  A  typical  experiment  with  the  N/32  solu- 
tion is  given  below : 

H2  —  Ptgauze  +  6  —  N/32  C6H5NH2.HC1  —  satd.  NH4NO3  — 
o.iNKCl  — HgCl  —  Hg 

Electromotive  force 
Time  14  :  gauze  14  :  6 

12.06  P.  M.  Started 

1 2.  1 1  P.M.  0.5021  0.4995 

12. 55  P.M.  0.5058  0.5042 

1. 58  P.M.  0.5067  0.5055 

2.  15  P.M.  0.5067  0.5058 

3. 10  P.M.  0.5070  0,5062 

The  remainder  of  the  aniline  hydrochloride  was  further 
purified  by  precipitating  it  from  alcohol  by  the  addition  of 
ether.  The  material  was  filtered  off  and  washed  with  ether 
to  remove  all  traces  of  alcohol.  The  salt  was  dried  in  vacua 
over  solid  caustic  potash  and  sulphuric  acid.  This  material 
constituted  Sample  II.  The  hydrolysis  of  N/i6  and  N/32 
solutions  of  this  salt  was  determined  as  before  except  that 
experiments  were  also  performed  with  potassium  chloride 
solution  to  eliminate  the  contact  potential.  The  summary 
of  the  results  is  given  in  the  following  tables: 


E.  M.  F.  with  NH4NO3 


-      E.  M.  F.  with  KC1 


Cone. 
N/I6 

N/i6 
N/32 
N/32 


Corr.  for 
calomel 

[H'  cone.'}  X  100 

electrode 

[total  salt} 

0.5005 

2.92 

0.4999 

2-99 

0.5009 

2.88 

0.500} 

2.88 

0.5126 

3-63 

0-512; 

3-62 

Corr.  for 

calomel 

electrode 


0.5086 

0.5095 
0.5l6l 
0.5168 
0.5189 
0.5190 


[Hf  cone.}  X  100 
[total  salt] 


2.13 
2.06 


(3-10) 

2.86 
2.85 


46 

The  results  obtained  in  the  experiments  in  which  potassium 
chloride  was  used  to  eliminate  the  contact  potential  agree 
much  more  closely  with  the  results  obtained  by  Bredig  and  by 
Denham  than  do  those  in  which  ammonium  nitrate  was  used. 

The  remainder  of  the  aniline  hydrochloride  was  recrys- 
tallized  from  acetone  and  the  product  washed  thoroughly  with 
ether.  After  it  was  dried,  solutions  were  prepared  from  this 
Sample  III.  Only  potassium  chloride  was  used  in  the  contact 
solution  with  this  sample.  The  results  are  summarized  below : 

E.  M.  F.  with  KC1 

Bar.  pres.          Corr.1  for  Corr.  for       [Hr  cone.]  X  100 

Cone.  in  ats.       calomel  electrode  bar.  [total  salt] 

N/i6  i. or i         0.5088         0.50852         2.15 

O . 5092  O . 50892  2 . 1 1 

N/i6          1.002         0.5091         0.50905         2.09 

0.5089  0.50885  2.  I  I 

N/32  i.  on  0.5184  0.51812  2.94 

0.5190  0.51872  2.8> 

N/32  1.002  0.5184  0.51835  2.91 

0.5184  0.51835  2.91 

The  agreement  between  the  results  of  the  two  experiments 
at  each  concentration  is  very  good  in  this  series  and  further- 
more the  results  agree  well  with  those  obtained  with  Sample 
II  when  potassium  chloride  was  used  in  the  contact  solution, 
if  the  one  experiment  be  excluded  in  which  the  values  3.18 
and  3 . 10  were  obtained  for  the  N/32  solution. 

A  typical  experiment  in  which  potassium  chloride  is  used 
in  the  contact  solution  is  shown  below: 

H2— Ptgauze  +  6— N/i6  C6H5NH2.HC1  (III)— satd.  KCl-o.  r  N 
KC1  —  HgCl  —  Hg  (No.  14) 

Time  14  :  gauze  14  :  6  6  :  gauze 

8.42  A.  M  Started 

9.52  A.  M.  0.5088         0.5048 

IO.I3  A.M.  0.5090  0.5072 

10. 28  A.M.  0.5090  0.5079 

10.58  A.  M.  0.5091  0.5085  0.00051 

12. 08  P.M.  0.5091  0.5089  0.00017 

1  It  is  worthy  of  note  that  the  averages  of  the  electromotive  force,  and  of  the  per 
cent,  of  hydrolysis,  per  cent,  of  ionization,  and  other  factors  depending  upon  the  elec- 
tromotive force,  are  approximately  the  same  for  long  time  periods  whether  corrected 
for  the  barometric  pressure  or  not.  The  fluctuations  of  the  barometer  here  in  Balti- 
more are  such  that  the  pressure  averages  close  to  760  mm.  over  long  time  periods. 


47 

A  fresh  lot  of  aniline  hydrochloride  was  prepared  and  was 
extracted  four  times  with  about  200  cc.  of  boiling  acetone. 
The  material  remaining  was  well  washed  with  ether,  dried, 
and  used  as  Sample  IV: 

E.  M.  F.  with  KC1 


Bar.  pres. 

Corr.  for 

Corr.  for     [//'  cone.]  X  100 

Cone. 

in  ats. 

calomel  electrode 

bar. 

[total  salt] 

N/i6 

0.987 

0.5087 

0.50904 

2.  10 

0.5083 

0.50864 

2.13 

N/i6 

0.989 

0.5100 

0.51028 

i.   9 

0.5100 

0.51028 

1.99 

N/i6 

I  .009 

0.5103 

0.51007 

2.01 

0.5100 

0.50977 

2.04 

N/32 

0.996 

0.5182 

0.51830 

2-93 

0.5180 

0.5l8lO 

2-95 

N/32 

I  .009 

0.5180 

0.51777 

2.98 

0.5179 

0.51767 

2.99 

Sample  V  of  the  aniline  hydrochloride  was  prepared  by  re- 
crystallizing  some  of  Sample  IV  from  alcohol  by  the  addition 
of  ether.  It  gave  the  following  results: 


E.  M.  F.  with  KCl 


Bar.  pres. 

Corr.  for 

Corr.  for 

[Hr  cone.  IX  100 

in  ats. 

calomel  electrode 

bar. 

[total  salt} 

I  .009 

0.5098 

0,50957 

2.06 

0.5098 

0.50957 

2.06 

0.996 

0.5086 

0.50870 

2.13 

0.5094 

o  -  5095° 

2.O6 

0-997 

0.5102 

0.51028 

I.99 

0.5097 

0.50978 

2.O4 

0.988 

0.5173 

0.51761 

3  oi 

0.5173 

0.51761 

3.01 

0.998 

0.5177 

0.5^75 

2.99 

0.5177 

0.51*775 

2-99 

0.999 

0-5183 

0.51832 

2-93 

0.5180 

O.5I802 

2.96 

Cone. 

N/i6 

N/i6 
N/i6 
N/32 
N/32 
N/32 


Sample  VI  was  prepared  by  recrystallizing  the  remaining 
material  of  Sample  V  from  alcohol  by  the  addition  of  ether. 
The  results  obtained  with  this  material  are  given  in  the  fol- 
lowing table: 


48 

E.  M.  P.  with  KC1 


Cone. 

Bar.  pres. 
in  ats. 

Corr.  for 
calomel  electrode 

Corr.  for     [H'  cone.]  X  100 

bar.                [total  salt] 

N/i6 

0.997 

0.5096 

0.50968            2.05 

0.5092 

0.50928            2.09 

N/i6 

0-995 

O.5IOO 

0.51013            2.01 

0.5097 

0.50983            2.04 

N/32 

0-995 

0.5189 

0.51903            2.85 

0.5187 

0.51883            2.87 

N/32 

i  .003 

0.5188 

0.51872         2.88 

A  summary  of  the  results  obtained  with  the  different  sam- 
ples of  aniline  hydrochloride  when  potassium  chloride  was 
used  as  the  contact  solution  is  given  in  the  following  table: 

Aniline  tfydrochloride 


N/16 

IH'  cone.]  X  100 

N/32 

[H'  cone.]  X  100 

Sample 

[total  salt] 

Sample 

[total  salt] 

II 

2.13 

II 

(3-18) 

2.06 

(3-10) 

III 

2.15 

2.86 

2.  II 

2.85 

2.09 

III 

2.94 

2.  II 

2.88 

IV 

2  .  IO 

2.91 

2.13 

2.91 

1.99 

IV 

2-93 

i  -99 

2-95 

2.01 

2.98 

2.04 

2.99 

V 

2  .06 

V 

3.01 

2.06 

3.01 

2.13 

2.99 

2  .O6 

2-99 

i  99 

2.04 

2-93 

VI 

2  .05 

2.96 

2.09 

VI 

2.85 

2.01 

2.87 

2.04 

2.8S 

2.07 

Average 

2-93 

0.08 

Max.   variation 

from 

49 

Excluding  the  values  in  the  parentheses,  we  have  the  aver- 
age value  of  2  .07  for  the  N/i6  solution  and  2 .93  for  the  N/ 32 
solution.  This  gives  us  the  following  per  cent,  of  hydrolysis 
of  N/i6  and  N/32  solutions  of  aniline  hydrochloride  if  we  use 
0.944  and  0.960  as  the  degree  of  ionization  of  N/i6  and  N/32 
solutions  of  hydrochloric  acid. 

2.07/0.944  =  2.19  per  cent,  hydrolysis  of  N/i6  aniline 
hydrochloride. 

2.93/0.960  =  3.05  per  cent,  hydrolysis  of  N/32  aniline 
hydrochloride. 

C6H5NH3C1     ±5;     C6H5NH3+  +  Cl~ 

The  per  cent,  of  ionization  of  aniline  hydrochloride  can  be 
calculated  from  conductivity  data  given  by  Bredig.1  From 
these  data  the  ionization  of  N/32  aniline  hydrochloride  is  86.6 
per  cent,  and  by  extrapolation  the  ionization  of  the  N/i6 
solution  is  found  to  be  84 . 4  per  cent. 

Let  kw  =  the  ionization  constant  of  water  and  kb  =  the 
affinity  constant  of  aniline. 

C6H5NH3+     5±     C6H5NH2  +  H+ 

__      [CH+] 2 


Cc6H5NH3+  =  Cc6H6NH3ci  X  ionization 

(i  — per  cent,  hydrolysis/ 100)  X  ionization 
volume 

k^    _  [CH+]2  X  volume 

kb          (i — per  cent,  hydrolysis/ioo)  X  ionization 
For  V  =  16 

kw       _    (O.O207)2 

kh~     "   (1—0.0219)  X  0.844  X  16  " 
For  V  =  32 

kw  (o.0293)2 


(i — 0.0305)  X  0.869  X  32 


=  0.0000318 


~4 


Average  value  of  kw/kb  =0.321   X  10 

Tizard2  obtained  by  colorimetric  methods  the  value  o .  242  X 

1  Z.  physik.  Chem..  13,  191  (1894). 

2  J.  Chem.  Soc.,  98,  2492  (1910). 


50 

io~4  for  kv)jkb  while  Bredig  found  0.24  X  io~4  by  his  conduc- 
tivity measurements.  Two  factors  enter  into  the  explanation  of 
the  difference  between  these  results.  The  first  point  is  that 
Tizard  assumes  that  the  hydrochloric  acid  formed  by  the  hy- 
drolysis of  the  aniline  hydrochloride  is  entirely  dissociated. 
As  was  pointed  out  hi  the  discussion  of  Denham's  work,  this 
assumption  is  not  justifiable.  The  second  factor  has  to  do 
with  the  calculation  of  my  results.  It  was  pointed  out  in  the 
discussion  of  Denham's  work  that  the  value  assigned  to  the 
potential  of  the  calomel  electrode  plays  a  large  part  in  the 
value  found  for  the  nydrogen  ion  concentration.  This  indi- 
cates that  the  greatest  source  of  uncertainty  in  the  deter- 
mination of  the  hydrolysis  of  aniline  hydrochloride  is  not  in 
any  difficulty  in  the  experimental  measurements,  as  Desha 
thought,  but  in  the  determination  of  the  potential  of  the  calo- 
mel electrode.  If  the  value  of  the  decinormal  calomel  elec- 
trode is  taken  as  0.3362,  the  figure  adopted  by  Desha,  the 
ratio  [H'  cone.]  X  ioo/  [total  salt]  found  for  N/i6  aniline 
hydrochloride  becomes  1.85  instead  of  2.07.  If  Lewis's 
value  of  the  contact  potential  between  o  .  i  N  potassium  chlor- 
ide and  o.i  N  hydrochloric  acid  is  adopted,  viz.,  0.0284,  the 
value  of  the  calomel  electrode  becomes  0.3355  and  the  ratio 
[H'  cone.]  X  ioo  /[total  salt]  becomes  1.81  for  the  N/i6 
and  2  .  56  for  the  N/32  solution. 
If  these  values  are  used 

kjkb  for  N/i6  =  0.247  X  io~~4 
0.242  X  io~~4 


This  gives  an  average  of  0.244  X  io~4  for  kw/kb,  which  agrees 
well  with  the  values  found  by  Tizard  and  Bredig.  It  should 
be  noted  that  Bredig  used  the  value  383  for  the  equivalent 
conductivity  of  the  hydrochloric  acid  formed  by  hydrolysis  of 
the  aniline  salt,  whereas  Lewis  used  389.9.  This  makes  no 
appreciable  difference  in  kw/kb.  Bredig's  equivalent  conduc- 
tivity should  change  with  change  in  concentration  according 
to  the  isohydric  principle.  I  intend  to  redetermine  all  the 
data  needed  for  such  work. 


Experiments  'with  Acetic  Acid 

To  test  the  applicability  of  the  hydrogen  electrode  to  the 
determination  of  the  concentration  of  the  hydrogen  ions  in 
solutions  containing  weak  organic  acids,  a  series  of  experi- 
ments was  carried  out  with  acetic  acid. 

First,  two  experiments  were  carried  out  with  o .  5  N  acetic 
acid,  with  saturated  ammonium  nitrate  as  the  contact  solu- 
tion. 

H2  —  Pt  —  o .  5  N  CH3COOH  —  satd.  NH4NO3  —  o .  i  N  KC1  — 

No.  14 

This  series  gave  the  results : 

E.  M.  F.  Per  cent. 

Corr.  for  dissocia- 

calomel  electrode  tion 

0.4817  0.764 

0.4822  0.748 

With  0.25  N  acetic  acid  and  ammonium  nitrate,  the  results 
were  as  follows : 

B.  M.  F. 

Corr.  for  Per  cent, 

calomel  electrode  dissociation 

,  o . 4908  i . 069 

0.4905  1.081 

0.4905  I.O8l 

With  o .  25  N  acetic  acid  and  potassium  chloride  as  contact 
solution,  the  results  were  the  f ollowing : 

E.  M.  F. 

Corr.  for  Per  cent, 

calomel  electrode  dissociation 

0.4930  0.982 

0.4927  0.993 

The  difference  of  approximately  o .  i  between  the  per  cent, 
of  ionization  determined  with  ammonium  nitrate  and  that 
determined  with  potassium  chloride  is  evidently  due  to  the 
change  in  contact  potential  in  the  two  systems.  Judging 
from  the  previously  described  experiments  with  potassium 
chloride,  that  solution  is  the  better  for  eliminating  contact 
potential  and  therefore  the  per  cent,  ionization  of  O.25N 


52 

acetic  acid  is  probably  nearer  o .  985  than  i .  080 .  By  con- 
ductivity measurements  White  and  Jones1  found  at  25°  the 
dissociation  of  o.sN  acetic  acid  to  be  0.58  and  of  0.25  N 
acetic  acid  to  be  0.89  (calculated  by  interpolation). 

Effect  of  Neutral  Salts  upon  the  Dissociation  of  Acetic  Acid. — 
The  catalytic  action  of  neutral  salts  is  a  problem  upon  which 
a  great  deal  of  work  has  been  done.  The  literature  of  this 
field  and  the  principal  theories  have  been  summarized  by 
Acree.2  Besides  the  catalytic  action  of  neutral  salts  upon 
the  velocity  of  decomposition  of  diacetone  alcohol  by  alka- 
lis, cane  sugar  inversion,  etc.,  work  has  been  done  upon 
the  effect  of  neutral  salts  on  the  dissociation  of  weak  acids. 
In  the  study  of  this  problem  two  methods  have  been  used 
heretofore.  The  conductivity  method  has  been  applied 
by  Arrhenius3  and  the  colorimetric  method  by  Brunei  and 
Acree4  and  by  Szyszkowski.5  Arrhenius  studied  the  conduc- 
tivity of  acetic,  formic  and  phosphoric  acids  in  the  presence 
of  a  number  of  different  salts.  He  came  to  the  conclusion 
that  the  effect  of  small  quantities  of  neutral  salts  upon  the 
dissociation  of  the  acids  is  much  greater  at  the  higher  dilu- 
tions of  the  acids  than  at  the  lower;  whereas  if  the  amount 
of  salt  added  is  larger,  the  effect  is  nearly  proportional  to  the 
amount  of  salt  added.  The  addition  of  o.  125  N  sodium  chlor- 
ide and  o .  5  N  acetic  acid  increased  the  hydrogen  ion  concen- 
tration by  5  per  cent.  A  number  of  factors  enter  into  conduc- 
tivity measurements  which  make  these  results  uncertain. 
Among  these  factors  are  the  change  in  hydration  and  in  vis- 
cosity caused  by  the  salt;  the  possibility  of  double  compounds 
or  complex  ions  between  the  salt  and  acid;  the  relatively 
small  change  in  conductivity  due  to  any  change  in  the  hydro- 
gen ion  concentration  compared  to  the  conductivity  of  the 
salt  added ;  and  other  factors. 

Szyszkowski  based  his  method  upon  the  change  in  color  of 
methyl  orange  in  the  presence  of  weak  acids  caused  by  the 
addition  of  neutral  salts.  He  studied  acetic  and  carbonic 

1  Am.  Chem.  J.,  44,  159  (1910). 

*  Ibid.,  41,  457  (1909). 

3  Z.  physik.  Chem.,  1,  110;  11,  823;  31,  197  (1899). 

*  Am.  Chem.  J..  36,  120  (1906). 

*  Z.  physik.  Chem..  58,  420  (1907);  63,  421  (1908);  73,  269  (1910). 


53 

acids,  using  solutions  varying  from  0.0022  N  to  O.O46N. 
He  interpreted  his  results  to  mean  that  neutral  salts  greatly 
increase  the  hydrogen  ion  concentration  of  weak  acids.  So- 
dium chloride  apparently  increased  the  ionization  of  acetic 
acid  about  23  times.  It  should  be  pointed  out,  however, 
that  Kurt  Meyer  and  also  Hantzsch  have  shown  that  some 
dyes  unite  with  salts  and  form  still  more  deeply  colored  double 
compounds.  This  tends  to  throw  doubt  on  the  validity 
of  Szyszkowski's  conclusions  until  further  evidence  to  ^the 
contrary  is  presented. 

It  was  believed  that  the  hydrogen  electrode  would  prove 
serviceable  in  the  study  of  this  problem,  and  to  that  end  a 
series  of  experiments  was  performed. 

In  order  that  the  effects  of  potassium  chloride  and  ammo- 
nium nitrate  for  eliminating  contact  potential  might  be  com- 
pared, each  solution  of  acetic  acid  was  used  with  both  contact 
solutions.  The  results  are  included  in  the  following  table:  The 
solutions  were  prepared  by  mixing  o .  5  N  acetic  acid  with  an 
equal  volume  of  the  different  solutions  of  potassium  chloride. 
The  concentrations  given  below  are  the  concentrations  after 
mixing  the  two  solutions.  To  make  the  table  complete  the 
results  obtained  with  0.25  N  acetic  acid  alone  are  included. 
Only  the  corrected  electromotive  force  readings  are  given: 

NH4NO3  as  contact  soln.         KC1  as  contact  soln. 


E.  M.  F. 

COTT.  for 

E.  M.  F. 
corr.  for 

Solution 

bar. 

Dissoc. 

bar. 

Dissoc. 

o 

•25 

N 

Acetic 

o 

,49032 

I 

.089 

0 

.49308 

0. 

980 

o, 

49071 

I 

•073 

0 

.49296 

0. 

980 

0, 

49071 

I 

•073 

0.25 

N 

Acetic  +  o. 

05  N 

KC1 

0 

49152 

I 

.041 

0 

.49276 

O. 

990 

0 

.49192 

I 

.026 

o .  25  N  Acetic  +  o .  i  N 

KC1                                0.49271  0.992  0.49178  1.030 

0.49271  0.992  0.49178  1.030 
o .  25  N  Acetic  -f  o .  5  N 

KC1                               0.49445  0.925  0.48945  1.125 

0.49445  0.925  0.48945  1.125 
0.25  N  Acetic +  2.06  N 

KC1                               0.49837  0.797  0.48575  1.300 

0.49837  0.797  0.48575  1.300 


54 

The  results  of  this  series  of  experiments  are  uncertain. 
According  to  the  experiments  with  ammonium  nitrate,  the 
addition  of  a  neutral  salt  appears  to  decrease  the  hydrogen 
ion  concentration;  according  to  the  experiments  with  potas- 
sium chloride,  the  hydrogen  ion  concentration  appears  to  in- 
crease. The  difficulty  evidently  lies  in  the  contact  potential 
of  the  system.  I  tried  to  carry  out  some  experiments  in 
which  ammonium  nitrate  was  added  to  acetic  acid,  but  these 
were  unsuccessful  for  some  reason.  Instead  of  the  potential 
becoming  constant  within  about  two  hours  it  would  continue 
to  rise,  showing  a  decrease  in  the  hydrogen  ion  concentration 
of  the  solution.  This  may  be  due  to  a  reduction  of  the  ammo- 
nium nitrate  to  ammonia  by  the  hydrogen  in  the  presence  of 
platinum  black. 

If  we  assume  the  potassium  chloride  series  of  results  to  be 
the  more  accurate,  as  they  have  been  seen  to  be  in  other  ex- 
periments, then  the  results  are  not  dissimilar  to  those  ob- 
tained by  Arrhenius.  The  addition  of  o .  i  N  potassium  chlor- 
ide to  0.25  N  acetic  acid  increases  the  hydrogen  ion  concen- 
tration about  4.5  per  cent,  of  the  original  value.  I  shall 
extend  this  study  in  several  related  directions. 

SUMMARY 

1 .  If  we  use  o .  339  as  the  value  of  the  electrode  o .  i  N  KC1 — 
HgCl — Hg,  the  hydrolysis  of  a  N/i6  solution  of  aniline  hydro- 
chloride  is  2.19  per  cent,  while  that  of  the  N/32  solution  is 
3.05  per  cent.     If  we  use  0.3355,  calculated  from  the  data 
of  Lewis,  as  the  value  of  this  electrode  the  hydrolysis  be- 
comes i. 8 1  per  cent,  for  the  N/i6  and  2.56  per  cent,  for  the 
N/32  solutions,  values  which  agree  excellently  with  those  of 
Bredig  and  Tizard.     The  hydrogen  electrode  gives  us  then 
another    instrument    for    studying    these    relations    between 
conductivity  and  hydrolysis  accurately,  and  we  shall  extend 
these  studies  to  a  large  number  of  other  organic  salts. 

2.  The  addition  of  potassium  chloride  to  acetic  acid  solu- 
tions slightly  increases  the  dissociation  of  the  acetic  acid. 


55 

BIBLIOGRAPHY 

1.  Abegg  and  Gumming:  Z.  Elektrochem.,  13,  17  (1907).     Elimination 

of  Liquid  Potentials. 

2.  Acree:  THIS  JOURNAL,  41*  457  (1909).     Studies  in  Catalysis. 

3.  Arrhenius:  Z.  physik.  Chem.,  31,  197  (1899).     Change  in  the  Strength 

of  Weak  Acids  by  the  Addition  of  Salts. 

4.  Arrhenius:  Ibid.,  i,  1 10  (1887).     Effect  of  Neutral  Salts  on  the  Saponi- 

fication  of  Ethyl  Acetate. 

5.  Arrhenius  and  Shields:  Ibid.,  u,  823  (1893).     Electrolysis  of  Alkali 

Salts. 

6.  Bancroft:  Ibid.,  10,  387  (1892).     Oxidation  Elements. 

7.  Barmwater:  Ibid.,  28,  424  (1899);  45,  557  (1903);  54,  225  (1906). 

Conductivity  of  Mixtures  of  Electrolytes. 

8.  Bjerrum:  Z.  Elektrochem.,  17,  58  (1911).     The  Reliability  of  Planck's 

Formula  in  Determining  Contact  Potential.  Ibid.,  17,  389.  On  the 
Elimination  of  Contact  Potentials  in  the  Measurement  of  Electrode 
Potentials.  Z.  physik.  Chem.,  53,  428.  Elimination  of  Contact 
Potential  between  Two  Dilute  Aqueous  Solutions  by  the  Introduc- 
tion of  a  Concentrated  Solution  of  Potassium  Chloride. 

9.  Bose:  Z.  physik.    Chem.,    34,    742    (1900).     Electromotive  Activity 

of  Elementary  Gases. 

10.  Bredig:  Ibid.,  13,  289  (1894).     Affinity  Constants  of  Bases, 
u.  Bredig:   Ibid.,  13,  191  (1894).     Contributions  to  the  Stoichiometry  of 

Ionic  Mobility. 

12.  Bredig  and  Fraenkel:  Z.  Elektrochem.,  n,  525  (1905);  Z.  physik. 

Chem.,  60,  202  (1907).     Hydrogen  Ion  Catalysis. 

13.  Bronsted:  Z.  physik.  Chem.,  65,  84  (1909).     The  Electromotive  Force 

of  the  Ha — O2  Element. 

14.  Brunei  and  Acree:  THIS  JOURNAL,  36,  120  (1906).     On  a  New  Method 

for  the  Preparation  of  Standard  Solutions. 

15.  Bruner:  Z.  physik.  Chem.,   32,    133  (1900).     The  Hydrolysis  of  Salt 

Solutions. 

16.  Coggeshall:  Ibid.,  17,  62  (1895).     Constancy  of  Calomel  Electrodes. 

17.  Denham:  J.  Chem.  Soc.,  93,  41   (1908).     The  Electrometric  Deter- 

mination of  the  Hydrolysis  of  Salts 

18.  Desha:  Diss.,  Johns  Hopkins  Univ.,  1909. 

19.  Desha:  THIS  JOURNAL,  41,  152  (1909).     An  Apparatus  for  the  Puri- 

fication of  Mercury. 

20.  Euler:  Z.  physik.  Chem.,  32,  357  (1900).     Neutral  Salt  Catalysis. 

21.  Freundlich  and  Makelt:  Z.  Elektrochem.,  15,  161  (1909).     Absolute 

Zero  of  Potential. 

22.  Gewecke:  Z.  physik.  Chem.,  45,  685  (1903).     Decomposition  of  Mer- 

curous  Chloride. 

23.  Goodwin:  Ibid.,  13,  583  (1894).     Study  of  the  Voltaic  Cell. 


56 

24.  Henderson:  Ibid.,  59,  118  (1907);  63,  325  (1908).     Thermodynamics 

of  Liquid  Elements. 

25.  Hildebrand:   J.   Am.   Chem.   Soc.,   31,   933   (1909).     Purification  of 

Mercury. 

26.  Hulett  and  Minchin:  Phys.  Rev.,  21,  388  (1905).     The  Purification 

of  Mercury. 

27.  Jones  and  White:  THIS  JOURNAL,  44,  159    (1910).     Conductivity  of 

Organic  Acids,  Etc. 

28.  Kistiakowsky:  Z.  Elektrochem.,  14,  113  (1908).     A  Method  of  Meas- 

uring Electrode  Potentials. 

29.  Laurie:  Z.  physik.  Chem.,  64,  615  (1909).     Electromotive  Force  of 

Iodine  Concentration  Cells  in  Water  and  Ethyl  Alcohol. 

30.  Le  Blanc:  Ibid.,  12,  351  (1893).  Electromotive  Force  of  Polarization. 

31.  Lewis:  Ibid.,  55,  449  (1906).     Silver  Oxides  and  Suboxides. 

32.  Lewis:  Ibid.,  63,  171  (1908).     Calculation  of  Ion  Concentrations  from 

the  Electromotive  Force  of  Concentration  Elements. 

33.  Lewis  and  Rupert:  J.  Am.  Chem.  Soc.,  33,  299  (1911).     The  Poten- 

tial of  the  Chlorine  Electrode. 

34.  Lewis  and  Sargent:  Ibid.,  31,  362  (1909).     Potential  of  the  Ferro- 

ferricyanide  Electrode. 

35.  Lewis  and  Sargent:  Ibid.,  31,  363  (1909).     Potentials  between  Liquids. 

36.  Lorenz:  Z.  Elektrochem.,  14,  781  (1908);  15,  157,  206,  293,  349,  66 1 

(1909).     Oxide  Theory  of  the  Oxygen  Electrode. 

37.  Lorenz:  Ibid.,  15,  62,  121  (1909).     Zero  of  Electrochemical  Potential. 

38.  Lorenz  and  Bohi:  Z.  physik.  Chem.,  66,  733  (1909).     Electrolytic  Dis- 

sociation of  Water. 

39.  Lorenz  and  Mohn:  Ibid.,  60,  422  (1907).     The  Neutral  Point  of  the 

Hydrogen  Electrode. 

40.  Loven:  Ibid.,  20,  593  (1896).     Theory  of  Liquid  Elements. 

41.  Lunden:  J.  chim.  phys.,  5,  574  (1907).*  Dissociation  of  Water. 

42.  Luther  and  Michie:  Z.  Elektrochem.,  14,  826  (1908).     Electromotive 

Force  of  Uranyl-Urano  Mixtures. 

43.  Maitland:  Ibid.,  12,  265.     Concerning  the  Iodine  Potential  and  the 

Ferri-Ferro  Potential. 

44.  Michaelis  and  Rona:  Ibid.,  14,  251  (1908).     On  the  Determination  of 

Hydrogen  Ion  Concentrations  by  Indicators. 

45.  Nauman:   Ibid.,   16,    191    (1910).     The   Electromotive   Force  of  the 

Cyanogen-Hydrogen  Element. 

46.  Nernst:  Z.  physik.  Chem.,  4,  150  (1889).      Electromotive  Force  Ef- 

fect of  Ions. 

47.  Nernst:  Ibid.,  56,  544  (1906).     Electromotive  Force  of  H2 — O2. 

48.  Neumann:  Ibid.,  14,  193  (1894).     Concerning  the  Potential  of  Hydro- 

gen and  a  Metal. 

49.  Ostwald:  Ibid.,  n,  521  (1893).     Dissociation  of  Water  Measured  by 

the  Acid-Alkali  Element. 


57 

50.  Ostwald:    Ostwald-Luther's    "  Physiko-Chemische    Messungen,"    3d 

Edition,  p.  441.     Calomel  Electrode. 

51.  Palmaer:  Z.  physik.  Chem.,  59,   129  (1907).     Absolute  Potential  of 

the  Calomel  Electrode. 

52.  Peters:  Ibid.,   26,   217   (1898).     Oxidation  and  Reduction  Elements 

and  the  Influence  of  Complex  Ions. 

53.  Planck:  Wied.  Ann.,  40,  561  (1891).     On  the  Difference  of  Potential 

between  Two  Dilute  Solutions  of  Binary  Electrolytes. 

54.  Richards:  Z.  physik.  Chem.,  24,  39  (1897).     Temperature  Coefficients 

of  Potentials  of  the  Calomel  Electrode,  Etc. 

55.  Richards:  Ibid.,  24,  53  (1897).     Temperature  Coefficients  of  Poten- 

tials of  the  Calomel  Electrode 

56.  Richards  and  Archibald:  Ibid.,  40,  385   (1902).     Decomposition  of 

Mercurous  Chloride  by  Dissolved  Chlorides. 

57.  Rothmund:    Ibid.,    15,    15    (1894).     Potential    Differences    between 

Metals  and  Electrolytes. 

58.  Salessky:  Z.  Electrochem.,  10,  204  (1904).     Concerning  Indicators  in 

Acidimetry  and  Alkalimetry. 

59.  Salm:  Ibid.,  10,  341  (1904).     Determination  of  the  Hydrogen  Ion  Con- 

centration of  a  Solution  by  the  Help  of  Indicators. 

60.  Sammet:  Z.  physik.  Chem.,  53,  673  (1905).     The  Potential  of  the 

Iodine  Ion  Electrode. 

6 1.  Sauer:  Ibid.,  47,  146  (1904).     Standard  Electrodes. 

62.  Schoch:  J.  Am.  Chem.  Soc.,  26,  1422  (1904).     A  Study  of  Reversi- 

ble Oxidation  and  Reduction  Reactions  in  Solutions. 

63.  Schoch:  Ibid.,  29,  314  (1907).     The  Electrolytic  Deposition  of  Nickel- 

Zinc  Alloys. 

64.  Schoch:  THIS  JOURNAL,  41,  232  (1909).     The  Behavior  of  the  Nickel 

Anode  and  the  Phenomena  of  Passivity. 

65.  Schoch:  Ibid.,  41,  208  (1909).     The  Electromotive  Force  of   Nickel 

and  the  Effect  of  Occluded  Hydrogen. 

66.  Schoch:  J.  Phys.  Chem.,  14,  719  (1910).     Behavior  of  Iron  and  Nickel 

Electrodes  in  Various  Electrolytes. 

67.  Schoch:  Ibid.,  14,  665  (1910).     The  Potential  of  the  Oxygen    Elec- 

trode. 

68.  Smale:  Z.  physik.  Chem.,  14,  577  (1894).     Studies  on  Gas  Elements. 

69.  Spohr:  Ibid.,  2,    194  (1888).     Effect  of  Neutral  Salts  on  Chemical 

Reactions. 

70.  Szyszkowski:  Ibid.,  58,  420  (1907);  63,  421  (1908);  73,  269  (1910). 

Contribution  to  the  Knowledge  of  Neutral  Salt  Action. 

71.  Tizard:   J.  Chem.  Soc.,  97,  2477.     The    Colour  Changes  of  Methyl- 

Orange  and  Methyl-Red  in  Acid  Solution.     Ibid.,  97,  2492  (1910). 
The  Hydrolysis  of  Aniline  Salts  Measured  Colorimetrically. 

72.  Tower:  Z.  physik.  Chem.,  20,   198  (1896).     Potential  Difference  at 

the  Contact  Surface  of  Dilute  Solutions. 

73.  Wilsmore:  Ibid.,  35,  296  (1900).     Electrode  Potentials. 


BIOGRAPHY. 

The  author  was  born  at  Grand  Rapids,  Wisconsin,  March 
1 6,  1888.  He  received  his  preparatory  education  in  the  public 
schools  of  Wisconsin,  in  the  Windsor  Township  High  School, 
and  in  Rochester  Academy,  from  which  he  graduated  in  1904. 
That  fall  he  entered  Beloit  College,  from  which  he  graduated 
in  1908  with  the  degree  of  B.S.  The  following  year  was  spent 
in  graduate  study  at  Syracuse  University,  at  which  he  received 
the  degree  of  M.S.  in  June,  1909.  In  October,  1909,  he  en- 
tered upon  graduate  work  in  Johns  Hopkins  University. 
Here  his  major  subject  was  Chemistry,  his  subordinate  sub- 
jects Physical  Chemistry  and  Geology.  During  the  year 
1910-1911  he  has  held  a  University  fellowship. 


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